Ok so I was looking to multiclass a Necro Wiz. and a Death domain Cleric going for a level 10/10 (level 20 total) Split for a one shot. While creating the character on Dnd Beyond it wouldn't let me prepare higher than 5th level spells for either class, even though it states in each class that I can prep spells for which I have spell slots for. It still gives me the spell slots and cantrips accordingly for a Multiclassing full caster, just can't prep higher than 5th level spells Can someone help either explain it or Contact Dnd Beyond to fix this bug/issue. 

As a 10th level wizard or cleric you only know up to 5th level spells. That’s why you can’t prepare spells higher than that. Even though you have 9th level slot at level 20. 

but rules for each class say that i can prep spells with spell slots i have available

Quote from Woodlandeye >>

but rules for each class say that i can prep spells with spell slots i have available

And the rules for multiclassing say you prepare spells for each class individually as if you only had those specific class levels.

From the spellcasting rules for multicasting (https://www.dndbeyond.com/sources/basic-rules/customization-options#Spellcasting)

Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.

So you can only prepare the spells of a 10th level wizard and the spells of a 10th level cleric.