In a symmetric square, your split box contains two boys and two girls.  One of the boys will be a head and one a side.  One of the girsl will be a head and one will be a side.  Such is the stricture of symmetry.

We can further characterize things thus:

1. Each boy's partner is in the box.  Not necessarily next to, not necessarily facing, etc., but there.  Partner box is a special case of this.  (Let's call this a PP half or group,.)
2. Each boy's opposite is in the box.  Their partner is not in the square.  (OO group.)
3. Each boy's corner is in the box.  For one boy, his partner is in the box,,anf for the other, his opposite lady is in the box.  Corner box is a special case of this.  (CC group.)
4. Each boy's right hand lady is in the box.  For one boy, his partner is in the box,,anf for the other, his opposite lady is in the box.  (RR group, or choo choo twain.)

There are no other possibilities.  You can convert each of these to any of the others by sending one or two selected people to the other box (and thus receiving their same sex opposites from the other box).

Callers school and various beginner literature and advice from experienced callers start by advising that you get into a corner box (an 8 chain thru setup example of item 3 state above, e.g.; from squared up, heads square thru), then use a bunch of calls that don't send any one to the other box, and that finally arriving back in the corner box arrangement, where you can call left allemande to resolve.

Your bunch of calls might (or might not) pass through the corner box state multiple times   The series of calls between one occurance of a corner box and the next is what they call a (geometric) zero, and they are very excited that you memorize a bunch of them (and it is a good idea), so that you can call them while deciding what to do next, (another zero, or a "get out" initially).

Eventually they point out that, at a zero box, you can call half of a chicken plucker or equivalent  (hereafter half CPE), which will convert you to a right hand lady box (my term), an example of item 4 above, where you can call some zeros, and then either another half CPE or a specially crafter get out from right hand lady box.

There are more variations on the theme, including line based schemes where you get into a partner line (item 1 above) and call split box zeros, then use a partner line get out.  (It's easy, for example, to convert partner line to corner box.)  In lines, the way you temporarily play with the other couple is pass thru, bend the line (still item 1), replacing half CPE used in the 8 chain thru setup case.  You can do it again to come back, or use a 2p1p or 1p4p get out.

You need to step outside these recommendations to ever be in an example of item 2 above.

When you start calling at actual dances, and you haven't broken through from the standards and zeros stage, you are skating on thin ice.  Some tip you will make a mistake "playing back" one of those zeros.

Note that if your mistake hasn't changed who is in your group, even if your group, at the moment, is some form of line rather than some form of box, or vice versa, that you can probably "wing it" with calls that don't move anyone to the other group, and find your way back to a corner box.  While doing this, you only need care about two dancers: a boy and his corner for corner boxes, or a man and his partner for 1p2p lines (so do take note of this wehn you arrive at the box, even if you didn't memorize 1 and 4 couples).  Just get them back together that way (facing corers or partnered partners) with all 4 in standard couples.  Then, if htey are on the wrong side, do a right and left through or something similar, and you are back in the familiar setup, and can either call more zeros, or take a breather by calling a suitable get-out.

Otherwise, if you have sent someone to the other group, you will have a more difficult time.  If you catch it right away, you may be able to "undo" it.  But that takes thinking, and you may make a mistake there, since you are unfamiliar with calling off script.  Pretty soon you're in a variation of item 2 above, or somewhere else that you don't recognize, and you are on the edge (or over the edge) of panic, and don't know what to do (probably best to bite the bullet and say "sorry, I messed up, everbody square up").

To borrow from Douglas Adams: Don't Panic!  Unless you don't remember who your key couples are.  In that case, panic.

If you can see your way to getting them into normal two faced lines or lines facing, Ed Foote has shared his easy methods to resolve from those two formations, no matter which of the items above applies.  They can be found in the Callerlab book for new callers, which has an additional resolve from normal lines.  I'll google to see if they're on line and add a link, or ask for permission to reproduce them here, or at least give you googling advice.  But if you're calling, you should have joined Callerlab, and can get the book, which has other useful stuff.

Or if you can see your way to getting them into a normal 8 chain thru setup, see my generic 8 chain thru resolve, or if you can get them into normal waves, see my spin chain through get-out.  Or find or develope othrers from whatever formation you desire.

There are analogues for this in, for example a general line of two facing lines considered to be half the square