wrote:

Doubling up two of these cables

If you mean using conductors in parallel on the same circuit, that is
generally frowned on.

heavy direct burial cable is expensive.

1.5mm 2 core SWA £0.88/m
2.5mm 2 core SWA £0.91/m
4.0mm 2 core SWA £1.20/m
6.0mm 2 core SWA £1.60/m

From http://www.tlc-direct.co.uk/Main_Ind...SWA/index.html
seems proportionately cheaper to me

Or is it best to have each cable on an mcb at the CU and no garage CU
with just two isolators as the cable enters the garage?

If you really want to use multiple cables then that would appear to be a
better way of doing it.