Hi congrats on getting a new puppy.

I also did this about two years ago, I had a female pup who was getting on great with her training, completely toilet trained, sat, lay down everything, then when she was about 7 months, I brought home a 21 wk old pup who had been abused since 8 weeks. It was a complete nightmare, my female pup completely turned around! She started weeing and pooing everywhere, wouldnt do anything she was told and when I let her off the lead, she wouldnt come back! I eventually had to pay hundreds to get a behaviourist in and she is ok now but it was very stressful and would never have two pups so young again!

I hope this doesnt put you off! Good Luck!