Many TCX Boeing crew at MAN fly the 757 and 767, but there's very little long haul on the 767 at the moment (although this could change this Winter).
A large number of TCX Airbus crew at MAN and LGW fly mixed fleet on the A320/1 and A330, however long haul is now much reduced with 3 x A330s about to leave the fleet, and so it's possible that the number of mixed fleet crew will be reduced as a result.
Although the mixed fleet lifestyle suits many, with a pilot surplus and a rapidly contracting long haul program I would say future prospects for new joiners being able to opt for this are pretty minimal.