Welcome aboard, Piraon! Congratulations on booking your very first Disney Cruise, and one to Alaska no less! That sailing is sure to be very magical, as that part of our country is certainly gorgeous. Sailing with DIsney Cruise Line will make that trip all the more special!
Once your window for booking
Port Adventures is open, (75 days prior to sailing for first time guests), you will be able to check the departure times under My Cruise Activities once you're logged in to your reservation. If you see two adventures with departure times that would work out, you could book both. The computer system will not allow you to book them both if they overlap at all.
I will caution you about booking two adventures on the same day, though. There is no guarantee on exactly what time your adventure will return. A rough estimate of time frame will be listed, but circumstances can always change the return time. A delay in return from the first adventure could easily cause you to miss your departure for the second adventure. It's really up to you, Piraon, but I would be too nervous about missing the second adventure to really enjoy the first. Once you are able to choose your adventures and view the exact departure times, you'll be able to get a better sense if this is doable for your family.
I hope that information helps. Let me know if you have any other questions before you set sail to the beautiful North with Disney, Piraon!