I don't think they'll purposefully be wanting to keep their past from the other- they're just both private people who don't really talk about themselves in general + probably some trauma from their past relationships that either might not be too happy to think back on. But I think as their relationship grows and they learn to genuinely trust and love each other, they might open up to their past if they feel like it, but they'll won't outright ask the other about it or pry.