Perhaps this is silly but here goes:

I have a pair of NT-1As which are fine for X-Y and ORTF recording, but I fancy doing a bit of work with a Mid/Side setup. Will a Rode NT2A in figure 8 config work well with an NT-1A? Or perhaps there's a cheaper solution - I was hoping that the Rodes would match well and justify the cost of investing in a NT2A.

I was intending to hang the NT1A upside down over the NT2A to minimise the physical distance between the capsules:

Yes mounting as you show is the best way to get the mics as coincident as is physically possible & prevents 1 mic shadowing the other. I have done this with a NT1a & 2a.
I used a pair of NT1a shock-mounts by removing the swivel part & bolting both mounts to a ally strip then using one of the swivel parts to attach the bar & mounts to a mic stand.

To prevent the possibility of the 2 mics banging into each other i put a small piece of double sided sticky pad between them.

Very bulky of course particularly if you need a windshield.
John L

Take care the mic hanging upside down doesn't slip from the shockmount while you're setting up. It can sit there looking secure for a while, then Mr. Murphy notices your back is turned...