WIth a good amount of homework first ...in many a smaller lot situation there could be at least two big issues ...and you do not want to raise red flags or create. A deal you are unable to complete .
Irrespective of lot size and separate tax bills, you are walking on eggs to try to unmerge two contigous lots merged via common ownerhip ...IF either of the two lots would be substandard as to any current codes .
IN some cases bright $$ idea seems to be to sell off ones own parking and garages ...and your home is out of code by a problem you created . In theory you should be denied a variance for a self created problem., ifmever a variance is needed.
ABsent any issues above, it it is likely a cake walk,for any attorney doing Land Development type work to get,separated,deeds,innplace ..but confirm first .
BTW! Were I still in Phila I'd explore other options first ...there is a nasty local transfer tax plus federal state and Phila tax on gain ..plus its loss of,control over what goes into that garage space. ..i.e. An ugly new building may hurt your values. A long term lease is deemed a transfer for PA tax purposes ...but it can be darn long and not be a tax trigger ...any yes you can lease out land and somebody else can build on it ...works even on small space ...not everybody's cup of tea...but the tea cup is out there..and on some deals "parking " is ultra valuable ...or the deal may not work !
BTW if people are knocking on your door to buy your garages ...I'll bet the space is a,whole,lot,more valuable than you think it is .
.