You will need to treat it as if your building two base weapon systems, then you will be able to select the master systems computer, and then link the second systems computer to it, thereby slaving it to the master computer, this of course disables the slave systems weapon out, and treats its weapon blocks as \"enhancers\"
The balance of weapon blocks in the slave/master system will affect the bonus, meaning you will not be able to add just 1 beam weapon block slave system to a 1,000,000 cannon weapon block master system, and expect to get a super sniper cannon... the benefit would be so small a range increase you wouldnt notice... this gets into the balance i mentioned.
Tentatively, you will want the master/slave computers to each have an even ratio of linked weapon blocks, to get the optimum bonus.