No, donations must be associated with only one member. However, it is possible to have both names show up on reports printed for both accounts. This is done by editing the Participant information in the Finance portion of LCR or the member can set this option themselves in the Settings part of the online Donations page. Reports for each account will still only show the donations for that account but all reports will have both names listed.
It is also generally possible to edit donations and change the donor if they want all donations on one account. This can be done in MLS by using the View/Update Donations menu option. But if this involves multiple donations then it can be a lot of work. And I'm not sure if online donations can be changed to a different donor.
Finally, each member can go to the Settings part of the online Donation page and check a box that will let their spouse view the donations on their account. Again, this doesn't merge the accounts but it lets each spouse see all donations and print reports for all donations but it still prints a separate report for each account.