Any problem in using two battery maintainers at once?

Easy. Connect them ALL to the strongest in the herd. Monitor voltage over a period of a week. If it droops then you'll need to fall back five and punt.

Personally I do not like tandem-ing high frequency devices. Like coupling an oboe with a cello. Even though a waltz may start, pretty soon they both regress where they don't belong - into jazz, and you in a heap o trouble. If you are a technocrit you can try adding a .47 uf capacitor - sort of like a midget conductor to the symphony.