Mix synthetic motor oil with synthetic?

Yes it will not.cause harm but you , might want to check the blend . Meaning that you may want to see if you are doing a 50/50 blend Ect

In order to ensure if you can mix these two syn oil, you should carefully check the base of each oil (cross check the oil formular). If you find they have same oil-base, you can mix it

Yes, according to Valvoline's web pages they are compatible. There are however some instances where fully sythetic oils should not be used, e.g rotary engines and wet clutches. So it is advisable to...

Yes, it is actually sold that way. It is called semi synthetic.

Who told you it is not recommended? You can mix synthetics as long as they are of the same weight. You can even mix synthetic with conventional oil.