I guess you can give it a try as we are all different.
If it was me, I would make sure I ran it at an easy pace and thought of it as time on my feet, i.e. taking roughly marathon time to complete the shorter 20m distance, rather than a test of pacing etc. 20 miles in 3:45 would be 11:15 min/mile which is probably too slow, but as your target pace for the marathon is 8:35/min/mile, around the 10:00 min/mile (max of 9:30 min/mile) would be the aim, i.e running for 3 hours and 20 mins as a target.
But only two long runs in marathon training doesn't sound enough to have built the necessary endurance base, so I suspect you'll suffer towards the end of the marathon regardless of whether you do the second Long Run or not.