Go for one or the other.
It is never a good idea to mix multigrade oils
The 20 W50 will be a little thicker when cold so may require you et the engine warm up fopr a few minutes before moving off.
If it is too thick you will hear the starter straining to crank the engine on really cold days.
The main reason why every one tells you their Honda push mower starts so easily is Honda use 10W 30 or 10W 40 oil so when cold you are pulling against the viscous drag of a 10W oil rather than a 30W oil.
And yes it does make a big difference.