Water Pressure to 2 bathrooms

I have two upstairs bathrooms. Recently, when the shower in bathroom 1 is on, there is no water pressure to the toilet in bathroom 2. If I open the cold water sink faucet in bathroom 2, the toilet gets pressure and refills. The faucet and tub in bathroom 2 have no pressure problems if the bathroom 1 shower is on. My questions are: what might be going on, and what can be done about pipes not getting proper pressure when water is flowing in others. In June, I did have shower hardware replaced in the bathroom that's losing pressure to the toilet. That's the only change in plumbing in 10 years.

Thanks.