Peeing in me bed!?!? HELP!!

Hi, my heeler is named frank and is 8yrs old. Just a few months ago he went from daily outside dog to inside apartment dog. He had trouble peeing everywhere at first but was still adjusting but now have no problems... we have another small dog Marley he is a mini Maltese poodle and they didn’t live together until now. Marley has accidents everywhere usually over night and we are trying to get a handle on it. So when we first saw pee on the bed we thought Marley. But it’s happened 3 times now and it’s ALOT of pee. So obviously frank( his bladder seems endless sometimes) he can be VERY protective of me as I’ve had him since I was 10. Is this a show of like hey this is my human type deal? Could it be he needs more exercise... I know he is very very active so I try my best to keep up with him. I don’t know what to do it’s only on the bed. Sorry this is very long but I wanted to paint the full picture.