That price is dirty cheap and building a second mortar would a far superior option than upgrading.
The price should be more like 200+150 in order to make sense.
If OP ideas was to be followed, i would agree with this.
I also think that half HP would be too low.
My concern is that the idea doesn't fix many of the intrinsicates problems with emplacements.
I'd keep it at say 280-300 and make the 2nd one fire as long as it's garrisoned. Adjust performance and popcap accordingly. Garrison bonuses are such an under rated element to balancing emplacements. Either they are trashy like now or fully autonomous and cancer like before. There has to be a middle ground and I think the extra resources of having to garrison is that middle ground.
Numbers aside, i agree. Having a unit inside, would add the man power bleed factor to a unit which is binary as vehicles.