In article .com,
"p" writes:
I want to use multicore cabel for domestic instalation ,but i dont
have 2,5 mm cable.
But I have only 4 core by 1.5 mm. Can I join the cores 2 by 2 to reach
a beter ampacity?

Not viable.

This would invoke the "conductors in parallel" part of
the regs, and it just isn't viable in the scenario you
describe as an alternative to buying the correct 2.5mm
cable in the first place.

--
Andrew Gabriel