is it okay to use a Y joiner so i can connect the single outlet of a reservoir to two inlets on the M/C or is this a NO NO?

Thanks.

As far as I know its a no no.

Each circuit needs an independant source so if one circuit blows (or the hose from the reservoir to the master cylinder leaks) the other circuit will not run dry.

Most stock reservoirs may only have one filler cap but are seperated into two compartments. They both fill together (and have one fluid warning sensor), but have seperate outlets and if ones leaks, the other is ok.

I believe doing that would fail SVA.
It's not safe IMO as you have a single point of failure to lose all hydraulic brakes!

okay can see the point that if one fails you would loose all fluid and other circuits may have problems, but how do production cars get away with it? or tandem reservoirs

See above - may be one container, but it's usually split into two separate compartments inside.