I am just repeating what Scientology itself SAYS - that you can be both a Scientologist AND a Catholic, Baptist, Methodist, Christian etc. According the THEM you do NOT have to "convert" or give up your "faith" - you can be BOTH - especially since Scientology rather leaves the whole "God/Bible" part OUT of their thing.
I am sure the Catholic Church might have something to say if challenged publicly but frankly (hubby is a "recovering" Catholic) so long as you continue to follow the teachings of the Catholic Church and attend Mass, Confession etc there is NO REASON the church would kick you out for taking Scientology classes and getting involved in the whole "Auditing" thing. Scientology isn't really a religion so much as it's a way of life and none of their teachings overtly contradict Biblical teachings or any beliefs of mainstream Christian religions. Scientology has some weird beliefs, (aliens and no pain killers or psychiatric meds), but none of them say you can't keep going to Mass on Sunday. OF COURSE they would prefer you be a PUBLIC Pure Scientolgist (al-la Tom Cruise) but if you're famous, they will take what they can get. Nicole Kidman NEVER renounced her Catholic faith and went to classes, auditing and was heavily involved in Scientology - and when she left Tom, she dropped Scientology and continued to be a Catholic - as she was all along WHILE she was doing Scientology. Tom didn't LIKE it, but Nicole continued to do both during their marriage. JLo and Marc certainly have the same options.
My Opinion