Dry hop multiple strains?

I just made a strong IPA (1.075), bittered with Magnum and flavored with cascade and centennial. I was thinking about dry hopping with an ounce each of cascade and centennial. Any thoughts?

We talking 5 gallons here? How much magnum and at what time(s). Really depends on what you're going for in terms of aroma/flavor...

They're classic dry hop partners. Centennial is "super Cascade" and more potent... i try to use less when paired with Cascade to get a better distribution of each profile. Id recommend 60-40 Cascade-Centennial.

5 gallons. Hop schedule was
.5 OZ magnum 60m
.5 OZ centennial 40m
.5 OZ centennial 20m
.5 OZ cascade 10m
.5 OZ cascade 0m

50 IBU

Oh and I'm looking for big citrus/grapefruit notes.

You've got more bittering hops than flavor hops there, and small amounts of flavor hops for an IPA.

Typically, you'll have hops at something like:
1 oz 60 minutes (bittering hops) to about 30-35 IBUs with this addition
1 oz 15 minutes
1 oz 5 minutes
1-2 oz 0 minutes
Dryhop with 2 ounces

So, you're pretty low on flavor and aroma hops. I'd hop with at least two ounces to make up for lacking in the late hops, and that should help alot. You probably aren't going to have "big citrus" with only 1.5 oz of late hops in the IPA. You can try to get more into it with the dryhopping, though.

You'll definitely get citrus/grapefruit notes from a combination of cascade and centennial. Given your batch size and OG I think you may want to use more dry hops. When I brew an IPA of that OG I usually use 4 -5 oz of dry hops and a minimum of 3 oz.

Ok so I'm going to get two ounces of each and maybe put in only 1.5-1.75 of the centennial.