Well, squadron transit would require both ships to have jump drives
I don't see why you think this. In squadron transit, only one ship per TG has its jump drive active; the squadron size of that jump drive determines how many ships can be in that TG. Since the squadron size is two here, the tug should be able to handle it. In particular, if you had two jump ships, each of whose jump drives had a squadron size of 4, among a total of 8 ships, then you would need to make a squadron transit with the 8 ships divided into two TG of 4 each (each with a jump ship of course). This is because the wormholes that the jump ships open will generally appear in different locations.
For the OP: I also checked that both ships are classified as "civilian", so that's probably not the problem.
The only thing I can think of is that maybe you've stumbled upon a freaky bug due to the fact that your jump drive has 126kton capacity but the ship is only 125kton.
As a side note, I would strong recommend having separate jump ships and tugs. The issue is that jump engines are expensive, and aren't being used when the ship is being used in a tug role.
John