Child support help

I have a 2 and a half year old son who I did nt have a relationship with for about a year because the mom did not allow it. Any time I tried to contact her to visit or to provide money, she ignored me. She finally decided to let me start coming around and I thought things were going to be ok, that we could now handle things as adults. Unfortunately, she ended up serving me with court orders to pay child support and they agreed and ordered back pay plus interest. I am now very far behind and owe $378 per month for current support/medical support and past due support/medical support. I had to take on 2 jobs because I am a full time student so I cannot have a full time regular hour job. My first employer received the income withholding notice and I have been participating in that for 2 months. It has not been enough to cover the full amount so I took on a second job. That employer just recieved an income withholding letter to withhold the exact same $378 per month. I called the CS office and they said that both orders have to stay in effect until one of them shows that it can pay the full amount. If neither shows that, both orders stay in place and take 50% of each pay check. That could potentially be more than the $378 ordered each month and the CS office said there is nothing they can do about that.
my question is..... are they allowed to do that? If possible, I would prefer the entire amount come from one job and the second job be strictly to pay my necessities. I am not trying to get out of child support or anything, but I don't think it is fair that I tried and she played me off like I didn't try and now I could be potentially paying over the ordered amount each month.
This case is in Texas, if that helps.