We had a couple of guys years back doing both and Fed Ex out of the blue said starting today you cannot work for UPS and FEDEX at the same time. FedEx let them go that day and one of the guys had 5 years in.
Another way to look at this dilemma. Say your out on route and a hottie housie wants some extra package, if you do it you gonna tell your wife/girlfriend? If you need to do both just do it and don't tell the other