Julibee...
NO... you can not join more than one. Pick one and it's your PRO for life. You can actually change if you want to, but it's not very often that anyone does. You can co-write with anyone from any other PRO without any problem..... but only ONE of the writers should register the song with their PRO..... it should not be double registered, as that simply causes problems that have to be sorted out.
There is no need to join a foreign PRO. Actually, I am not 100% sure, but I think you can only join a PRO in your country of residence. If you hook up with lets say BMI, and your buds are in some Swedish PRO, the proceeds will be split according to the laws in the country on performance and they will send the bux back to BMI who will make sure you are paid.
NOW.... the real question is if you even need to join a PRO.... PRO's are for composers and writers of songs. They are the people who collect the PERFORMANCE royalties from concerts, clubs, internet, TV and radio play of the music in their home country. They pay the writers and the publishers who are the owners and writers of the music. Generally a 50/50 split of the net.
An Artist, who is simply an artist does not really need to join the PRO since an artist would be paid for their performance in generally one of two other ways. Mechanical royalties would be paid to the artist from sales, which are paid by the record company, not the publisher or PRO. An artist might also be paid on a session basis. One set fee, signed off with a waiver of rights and done.
So unless you are a writer of the music, or the existing writer is giving you a cut of the ownership.... I don't believe a PRO membership would be useful to you.
Oh yeah: some other advice. no matter what.... do not pay anything out of your pocket..... it should not cost you a dime. If the request comes in for money to "help with this or that" run.
and..... for a songwriter....joining BMI, ASCAP, or SESAC is FREE.
I have been with BMI for many years. They all handle all genre's these days. Back in the day BMI used to be more country orientated..... but times have changed.