Will if it doesn't confuse the child, I'd think it would be nice to learn about 2 religions/faiths & then choose your own.
However, this must be a little confusing. You either believe in Christ or you don't. It's not like one is Baptist the other is Protestant...there's the same root belief there.
Ask you niece sometime if it confuses her. If not, then be happy for her!

"God and the soldier, all men adore
In time of danger, and not before.
When the danger is passed and all things righted,
God is forgotten, and the soldier slighted."
unknown soldier