How can I link my spouses tree to mine?

You cannot link your two trees in the private spaces that are in each of your own accounts. You have two options:

1) Create a minimum number of duplicate living individual in each of your accounts to link into the deceased branches of your families.

2) Create a Family Group Tree in which you copy the living individuals in each of your accounts into a common family space. This is a new feature that, while still in development, is available for use. See: https://www.familysearch.org/labs/