Deep cell and Regular battery

[CHIEF MK]

I was just wondering if I could hook up a regular car battery to the existing deep cell trailer battery in parallel. I mean I know I could do it, but would it do any harm to the charging system or anything else you could think of ? I have a good spare AGM type battery, and it was just something I've been think about.

I really will probably never go boon docking, but it would be a way to use this battery.