I'm about to change the diff fluid on my 05 LS430, I have a quart of Redline 75W90 and my dad has a quart of Motul gear 300 75W90 gear oil, was wondering if its fine to add both of them in or if its better to stick with one brand?(The diff holds 1.5 qts)

 

While the mixing may slightly degrade the performance capabilities of one relative to the other, there's not going to be any detrimental outcome which would harm the equipment.