He should be able to chose either SIM as the "data" SIM. At that point, the other SIM no longer makes a data connection to the wireless provider. But that SIM still should support voice and sms.
On my 950 XL, the SIM chosen as the data SIM will make an LTE connection, and the other SIM will drop to E or G. Both SIM still support texting and calling.
However, I too witness one or both SIMs struggle at times compared to having just one SIM in the phone at a time. For example, the data SIM might drop from LTE to 4G for a few minutes, and then recover to LTE. And the non data SIM will lose its connection for a few seconds and then recover to a weaker or stronger (bars) than it was when it dropped.
I'm betting since Microsoft released the dual sim model in such large numbers, it will be forced to further develop W10M's support of it. So as happy I am with this crazy cool mobile weapon, I suspect it will get even better. More refined.