Well, depends on what you mean by "legacy", and by "run two trains on one line". If by "legacy" you're referring to the earlier Lionel control system, I'll defer to someone familiar with that system. If instead you mean two engines using conventional control (varying the track voltage by transformer-based throttle to control speed and direction), then the answer is yes, sort of, with a few asterisks.
One method, as you suggest, is to just link and synchronize two similar engines (think A-B or A-A diesel units) so they both pull in the same direction at the same time. I've not personally run my trains in that fashion, but at least in concept it should be fairly straightforward, if a bit fiddly, once you work out the appropriate couplers.
OTOH, if you mean two separate trains, each with its own engine and consist, the answer is still yes, though needing some additional bits and pieces, all to the end of creating some form of block control system.
The very simplest I've seen is the control system I've been experimenting with in order to run two conventional trolleys on the same loop of track:
The system is an adaptation of a twin hand car system put together by Joe Rampolla:
The technology used in Joe's system and adopted in my projects drop-dead simple, though ingenious. Earlier, I had spent some time building an updated version of the early Marx "Twin Train" set, that used a signal-based relay to control two trains on a figure-8 track.
However, if you want to dive even deeper into conventional block control systems, here's an excellent video I just found that details not only the roots of such a system, but all the nuts and bolts you'll need to get into it, based on old-school Lionel components and info:
Good luck!
