Double would be only for the truly extreme flow needs. Radiators are not restrictive. Most restrictive thing you have are the gpu blocks. If you set them up in parallel, 1 D5 would do the job.
Although, you're at the limit I suppose of 1 D5. If you planned on adding a gpu or 2 then you would need 2.
I have 2x 7970's with EK vga blocks on. Not that they seem to restrictive.