If I want Dorico desktop and iPad. (Not the free versions) do I have to pay for both?
In short, yes! However you may not need everything that the paid version for ipad gives you. It really depends on your workflow and needs. For instance, you can enter the notes on ipad and finish up on a desktop version.
Here’s a thought for you: get the free ipad version and a trial for the desktop. Use them both and see what you need, what’s not included (ipad), and what you can live without. After trying out the desktop trial version, try a monthly subscription for ipad to see if it has all the bells and whistles you need (it doesn’t have everything the desktop does). This way you can minimize the financial impact and get what you truly need!
— Jim
[Edit] Welcome, btw!
Welcome!
I had the paid iPad version and seldom used it so I cancelled my subscription. If you use your iPad in place of say, a laptop, when traveling or whatever, you might want the full iPad version. If you have a laptop and willing to use that instead, you can put the desktop version on your laptop and your desktop with the single license purchase (up to 3 devices if I recall correctly).
@jaskarbong is correct with his answer that you would have to pay for both if you want the full iPad version.
That’s certainly a better option. The ipad version lacks the ability to edit page templates and other things. There are also problems loading some fonts… 
Dorico does allow 3 licenses as @kasky1 mentioned — you can use them on desktops, laptops, or any combination, whether PC or Mac or both.