You have to be very careful when deworming a horse. It is easy to give them too much and cause them to colic or develop other serious health problems.
Actually, it is quite difficult to make a horse sick with too much dewormer, unless you're giving a moxdectin or praziquantel dewomer. A couple of vet schools did field testing on the toxicity of dewormers. Moxidectin was the most potent, causing adverse reactions at four times the recommended dosage (in a healthy horse). Ivermectin was toxic after 10-15 times the recommended dose (that's TEN tubes of dewormer!!). They couldn't really get a toxic reaction using fenbendazole or oxibendazole, even at over 50 times the recommended dosage.
Now, if the horse has a large infestation of parasites and you give too much dewormer, you can risk the horse colicing from a large die-off of worms. But, the dewormer itself isn't what's causing the problem.
If I have a horse with unknown deworming history, or tested and showing a large number of parasites, I will deworm with a normal dose of Pyrantel, Ivermectin in two weeks, then Quest Plus 4 weeks after that. Pyrantel is only about 50% effective in a single dose in the Southern US, so it won't create a huge die-off.
As for double dosing, you can use two brands as long as they are the same ingredient. I give at the same time. I only double dose with Pyrantel as it will get some tape worms at double strength. Exodus has a "multi-dose" tube you can buy that has 2400 lbs worth of dewormer in it, so it works perfect.
OP: If you have a horse with a high infestation of worms, I would use the plan I detailed above. Oxibendazole and Fenbendazole are not effective, and are not very effective in the "power pack" dosing of 5-days of double strength dewormer. You're wasting money. Ivermectin followed in two weeks by Quest Plus will clean your horse out much better.