As far as I am aware you can see both at once if you drop into Glasgow at 13:30. 390 services are hourly however I think there's only 3 801's you can see. One in the early morning, the one around midday then one later on at night.
If you do the run on the 380 4 car, just after 1700 anti clockwise circle route. I think it was at 1717 You will see a 390 and a 350 on the platforms and just after the bridge you will be sanwiched by a 385 and 314 going thw opposite direction.