Unfortunately I don't have the space to house both machines individually at the moment, nor will I have a two car garage in the next 5 years or alternative situation that will allow me to have two machines.
This is usually the reason people consider these 2 in one machines. If you actually compare apples to apples the combo machines do not really save you money, and only save a little space.
The milling machine part of most of these combo machines is quite small, in most cases much smaller and less capable than something like a PM25 or Grizzly G0704, which are small mills.
https://www.precisionmatthews.com/shop/pm-25mv/
https://www.grizzly.com/products/grizzly-7-x-27-1-hp-mill-drill-with-stand/g0704
The 13x40 lathe on the Granite is also not really comparable to most 13x40 lathes. They raise the spindle higher than normal to gain swing (diameter). In reality it is a smaller lathe pretending to be a 13x40.
The Granite 1340 has a spindle bore of 1-1/8" which is quite small for a lathe in this size class. It also only weighs 750lbs including the milling head. Weight relates to rigidity. It is hard to really make a good comparison of the milling attachment since it uses the lathe cross slide as a table, but it falls somewhere between a large mini-mill and the PM25 / Grizzly G0704 I linked to above. Smithy says you need an area of 85x43" for the machine.
Most lathes with an 11 or 12" swing typically have a spindle bore of 1-1/2". Many of the 13x40 lathes have a spindle bore of 1-9/16". A 13x40 lathe will generally weigh around 1200lbs, a 12x36" around 900lbs (and that is all lathe, it does not include 100-200lbs of milling head).
Smithy says you need an area of 85x43" for the machine.
If you were to pair a mill the size of the PM25 / G704 with a 12x36 lathe (theoretically "smaller" but as I said above in reality you aren't giving up much) you would only be looking at perhaps another 24" in length, and possibly less depth.
Most 12x36 lathes are around 61x24", a small mill like the PM25 / G704 is listed as needing 57x25" but that is to provide room for full travel of the table. You can have a certain degree of overlap with other things since the table is well above the floor, and can be moved to one side or the other to allow access past or to get it out of the way. I use much of this dead space below the mill table for storage.
Worst case scenario total length 118" (9-1/2 feet so could probably fit the short way across a one car garage with a little room to spare). If things are really tight creatively locating the mill, or mounting it on wheels could probably reduce the needed space by least 12-18". Mounting the lathe and mill back to back would only require a space of 49 x 61", an overall increase of only 6" of depth and a reduction in length of 24".
One last plus to something like a 12x36 and small mill. When you do have more room, you can keep the lathe if it still meets your needs and upgrade the mill to something bigger if you want a bigger mill.
If you really want to go with a combo machine, I'd buy used. With the exception of the Emco lathe / mill combos (high end Austrian machines) combo machines do not hold their value very well. I see machines comparable to the Granite 1340 on Craigslist often for around $800-1200 which is less than most smaller single function lathes.
Agree with some of the other posters the Bolton with its stand alone milling head is a better design, it makes fewer compromises to the lathe than the Granite. I would hesitate to recommend Bolton though as they seem to be a low tier importer.
I know you didn't want to be talked out of a combo machine, but if you decide to get one you should fully understand what you are comparing it to.
