If you make a major fuss you might get fired for some strange reason so think before you mouth off.
In general your employer can change your pay rate going forward but not for work already done. If you don't like the new pay formula you need not volunteer for extra work .
The employer must pay OT once past 40 hours or whatever your state law addresses which might be done daily .
But management can use a variety of steps to limit paying OT such as
1. Set the rate for the second job at a lower rate and only pay OT based on lower rate when your time passes 40 hrs
2. Only schedule folks with less than 40 hr on regualr job for the seasonal job and then apply # 1
Paying T +1/2 is not a very effective way to staff most jobs so I don't fault management for applying a more cost effective formula ---but bottom line they must pay OT for hrs worked beyonf 40 or whatever --just they need not schedule people likely to trip into OP for this added work .
Its a bit of a 2 way street --and not every seasonal job is easy to fill w/o willing workers who know what they are doing.
Whether management can mandate that you do extra work at a lower pay scale is a bit beyond me....but they sure can mandate that you do extra work!