We have used a texel - gave nicely matched lambs, grew faster than pure Heb so ready Nov/Dec, more compact carcase and lovely skins for tanning. They got us a first prize at our local show for crossbred meat lambs
Oh and of course they are white.
We have also used a Jacob tup - again faster growing so ready in the autumn, as big as the Jacobs, interesting skins/spinning fleece, and they are black, sometimes with some white splashes.
You can use many different breeds of tup, but best not to choose one with too large a head (the texel we used was smaller in the head and shoulders than many). Hebs have a larger pelvis for their size than more commercial breeds, but still it's best not to use something huge.
Hebs are very milky (they were many crofters' milk supply) so have no problem in supporting faster growing crossbred twins.