Mortgage Question

Writing from TX.  I have a finance sort of question I hope people might be able to shed some light on…
5 years ago  I bought a small (1300sqF) townhouse, narrow but tall. It is one of 5 attached townhomes (all built in early 70s), but we are all “independent” insofar as there is no townhouse organization, etc.  Each of the 5 properties is a separate entity, though physically attached to each other (there are two larger end units, and three smaller units in-between, one of the smaller which I own).
In 1997 I closed at $135,000, 50K down payment, about 6.8% interest on a fixed 15 year note.  Mortgage payment sans insurance/tax escrow is about $800/month.  The mortgage is on the remaining $85K.
Now it is possible that one of the 2 units I am “connected to” via a brick wall might come up for sale within the next year. That unit is exactly identical to mine vis a vis size/layout/accomodations.  I am “guessing” it might list for $200-$225K (prices have shot up in my neighborhood.  I am thinking that if I also owned that unit, at some point I could easily knock out some walls, etc. and have a unit that is 2x my current size and would be quite “remarkable” (layout of these units is great, and we have a *superb* location). Anyway, is it possible (legal?) to “redo” my mortgage using equity in this unit to buy the 2nd one?  (Please forgive my ignorance I know literally nothing about these matters).
I am wondering if someone can, based on this limited information, crunch some basic numbers for me if this is possible?  Or lay out what this would involve if it is legal and doable.
A few more notes:  for tax purposes (i.e. homestead exemption) I would “merge” the addresses into one so that I can claim the entire combined property as my homestead (I think this can be done since the lot and the townhomes are quite literally attached to each other).  Since I have returned to school, I would for the time being rent out the acquired townhome, which I could probably get $1100- $1300/month for quite easily.
So, I am looking at my place with 5 previous years payments of $800ish/month, with the $50K originally down.  I have a 15 year note at 6.8%(ish).  The unit next door will probably sell for a bit over $200K. I could get $1200ish in monthly rental income for that place until I am employed again (i.e. not in school) and can renovate.
Given that info, is there a situation where I would essentially “break even” until I can do the extensive renovation and end the rental?  I guess what I am asking, is do I have enough equity in my current home, + rental income, to acquire that place for a net “wash” or close to it with a new single mortgage that covers both places?
I’ve *dreamed* about combining these 2 units for 5 years now, but the neighbor had no intention of ever moving.  But that situation has suddenly changed, and I want to know what my options would be should it hit the market (it will sell fast when it does).
Thanks for any thoughts you might have, whether this scenario is feasible or not.  I *really* appreciate input from anyone well versed in the real estate profession.