You'll want to create a custom bundle and apply that to the client ssl profile. This solution article discusses doing that with certificates that have already been imported:
https://support.f5.com/kb/en-us/solutions/public/13000/300/sol13302custom
You can also do this in the GUI if you have the certificates in the proper format (PEM). To do this, select "import" under File Management -> SSL Certificates List. Set import type to Certificate and select the "paste text" radio button. You can then paste the two certificates in one right after the other (single carriage return, no white space). After you've imported it make sure to click on your custom bundle from the SSL Certificates List screen to verify both are there and the certificate properties are what you expect.