Hi Bonzaigirl,
You could try pumping one at a time or use one medium and one large.
OR
Do you have clamps?...that would really help. You could clamp one tube on the Y (example the right),
then pump the left. when you get a good suction clamp the the left, release the right. Pump the right until you get suction. Then you can release the left clamp.
Initially I had trouble using both medium domes at the same time, so I ordered the clamp and pumped one at a time until the clamps arrived. Of course, by the time I received the clamps I was able to get suction with both domes at the same time.
It will work out...I'm sure.
All the best!