H Bridge vs Half Bridge current sense
Sure you can use only one shunt, but you will have the information of PSU current, only. The configuration of two shunts possibly can be used to estimate the current through the load, although I don't have any information about the existence of this calculation and software implementation, I do think that's the reason of using two shunts.
EDIT:
I do see now, you are tying to detect the current limit, only. I wrote my answer thinking you are using ADC to measure the current. So, it makes no difference to use a single shunt and single comparator for your needs. As for negative voltage on the shunt, you have to make sure that it will not exceed the comparator input voltage limits.