I understand that. But as you can see on the screenshot (https://gyazo.com/91fa822c4fbfcfd08787c4102872ff9d) I have only a single input field for specifying ETH hashrate.
So I don't understand how can I specify different ETH hashrate values for ETH+DCR and ETH+SIA pairs.

For example I have the following optimal values for my rig:
ETH+DCR: 27 Mh/s ETH, 750 Mh/s DCR, dcri 27
ETH+SIA:  24 Mh/s ETH, 580 Mh/s SIA, dcri 23

I cannot specify that in minerstat since I have only one input box for ETH hashrate. So I can pick only one ETH hashrate value, while I need a separate hashrate values for eash pair (ETH+dcr, ETH+sia, ETH+lbc etc). Does that make sense?