You can set them both up to use the same member files; however it can be major trouble on your memberdata since you have so many threads on one board than the other, if you were to rebuild them it'll throw the postcounts off. Charles will also say it's a bad idea due to throwing off the data.
Also, if their on two differant hosting accounts, the majority of hosts won't allow access via paths outside of your account due to security issues that can arise.
