In an air separation plant I have a large air drier with two compressors, one is bigger than the other.

The unit is controlled by Carel Pco5 and superheat controller. Each compressor has its own speed controller.

When the first compressor kicks in, it accelerates up to maximum speed and then the second compressor kicks in. At that point the pressure starts to decrease and the units trips on low pressure.

I am wondering about the sequence of operation regarding the speed of the compressors, should the first one slows when the second one kicks in and maintain the pressure or keep the first one at top speed?

The sequence of starting changes by 3 min resting time.
Refrigerant is 410A.
HGBP is present but I think it might be faulty.[Sporlan HGBE-8]

Any thoughts? Different ideas?