If you've knit the two rectangles separately, you can't knit them together. You can, however, use the mattress stitch to weave them together nicely. There's a video that demonstrates it.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry