Certified GKH Gardening Expert
Grafting bulbs together, usually, doesn't work too well. Something like this can be done in a sterile lab, but this is not feasible for the home environment.
You can plant them in a large container together, however.
Yellowing leaves can be a sign of many different issues, so it will be hard to say why they are yellowing. The usual suspects are not enough light, too much water, and pH issues, but this can also be a sign of actual nutrient deficiencies.
Here are some articles that will help: