The plants do look similar, at least their leaves, so it might be an interesting combination if you're trying to accomplish a foliar monotype with seasonal color differences (since clivia are spring-flowered and agapanthus summer). I personally prefer foliage contrast and like to see long, strap shaped leaves with leaves that are broad and either very large or very small. Perhaps you could use a few clivia/agapanthus as planter "backbones" but interplant with other perennials.

Plants with soft foliage would be nicely or intertwining plants..... here I'd plant linum perenne or moonbeam coreopsis for this effect or fennel .... but not sure what's appropriate for your region........ :)

Be careful of the agapanthus as it has a way of becoming invasive. When it flowers, the seeds have a way of blowing or dropping into all the other plants around the immediate area. What happens then is tons of tiny sprouts above the ground and thousands of miles of roots that strangle out all other life. Best way to control it is to cut the blooms before they die and reseed.I live in N. california and have just spent 2 weeks digging them out of a 20 x 40 foot area! They were a thought from the previous homeowner as a way of killing off the ivy. The ivy is still there. These just becme more work as I had to dig up and replant hundreds of plants because they were getting choked out.