This is a timely question and I have noticed that more and more triathletes are trying their hand at ultra events as they seek a different challenge. I do not see any reason why an athlete shouldn’t train for both events at the same time. Of course a lot depends upon what goals they have. Training specificity is very important but if you just want to have fun then why not?
Here is what I would suggest when training for an ultra. Firstly, you don’t have to run as many miles as you think. Most triathletes are completing 8-10 hours training each week all of which benefits the aerobic system and thats probably the same total (or more) compared to someone who only runs. The added benefit is that the legs wont be as fatigued (or injury prone) as those of a pure runner. Add one more run session, and reduce your bike and swim sessions to 1-2 per week to act more as active recovery. Maintaining your swim and bike fitness will allow you to easily switch back to a more conventional tri programme once your ultra event has been completed.
If you are training primarily for a triathlon, then your normal training pattern will allow you to maintain good fitness so that you can switch up to a bit more running when you choose to do an ultra.