[Baron Spikey]

No you can't, you can only have 1 unit in a transport unless the transport's description explicitly says otherwise. Independent Characters can join units in order to embark on the same transport but that's pretty much the limit for '2' units in 1 transport.

 

[Baron Spikey]

Hurmph. Didn't know that. Nothing like that ever came up, but I always assumed you can fill a transport to capacity...
What transports can fit multiple units?

Mainly stuff in Apocalypse, but the Storm Raven can transport a unit and a dreadnought which is the only transport in standard 40k that I can think of.

 

[Baron Spikey]

What ever choice you select it from, if it's chosen from the Elites section then it's an Elite, from the Heavy Support section means it's a Heavy Support choice etc.