If you disconnect or severely limit the signal at one aerial it will eventually switch to the other - putting a can over it or flipping it 90deg is often good enough. What I haven't seen is any convincing evidence it's ever using both simultaneously or spontaneously switching other than dealing with a terrible signal. As a diversity aerial it seems to just scrape the minimum definition as an alternate option after catastrophic signal failure.

There seems to be no way to directly check them, maybe because it's only using 1 at a time?

At least for transmitting it is only using one antenna in my case, even if both are attached (confirmed with spectrum analyzer and h-field probe).

When both antennas are connected, the MR600 shows 4G+ with bands 3,20.

When only one antenna is connected, the MR600 shows 4G with only band 20 (slow), and one port being much worse than the other.

I'm using a cross-polarized log-periodic antenna.

@Baenz makes sense being able to use both antenna for 4G+, especially between bands as far apart as 3&20, each one will have different progagation properties and the built in antenna very different reception due to spacing. B20 signal strength is unusable here, with any of the antenna I've tried (though my old Huawei could see it on internal antenna,just). Bit peculiar if an external antenna changes reception Q just by switching socket though.

I could simply be getting such different Q from each antenna the poorer one just can't help, still think they've cheaped out on diversity though.

[and now I think I'll have another play with cross polarisation again, nothing better to do ;)]

Situation here is B20 is strong but slow (-80 dBm, 5 Mbit/s), B3 weak but fast (-101 dBm, 75 Mbit/s).

But with a single antenna it doesn't make use of band 3 at all and always choses B20.