Lol I never knew there were so many C-Rank weapon Ninjutsus and I hope I don't have to learn more anytime soon 
I don't have a summoning yet because I have to get to B-Rank before I can actually summon an animal so I'm just going to wing it
One/three advantages:
#1) I'm assuming by combining, you would have access to your summining animals abilities
#2) Doesn't require any hand seals, you just have to sync your chakras so I think it would be quick to perform
#3)
One/three disadvantages:
#1) You have to be close to each other to perform this jutsu
#2) If you are hit with a jutsu then your opponent will be injuring both you and your summon(killing 2 birdies with 1 stone)
#3) You become ugly as shit xd
The 3rd disadvantage was just for shiz and giggles xd
How you would use it in battle: I wouldn't use it but if I did then I would use it when the summonings abilities has an advantage on the terrain(eg. Toads on a water terrain)
How you would counter it: I would try to attack them while they are fusing(naruto and gamabunta = narubunta/gamato xd) or I would use jutsus that are powerful but quick to perform so if I do hit them, I will be injuring them both
How you perform it (with as much detail as you can): I bite my thumb to release some of my blood and perform the required hand seals before slamming my hand on the ground, summoning my Cheetah(made up) which stands next to me. I then let the Cheetah in on my plan and we begin focusing chakra throughout our entire bodies, we then release our chakras from our bodies to combine and make us become 1 being

I don't have a summoning yet because I have to get to B-Rank before I can actually summon an animal so I'm just going to wing it
One/three advantages:
#1) I'm assuming by combining, you would have access to your summining animals abilities
#2) Doesn't require any hand seals, you just have to sync your chakras so I think it would be quick to perform
#3)
One/three disadvantages:
#1) You have to be close to each other to perform this jutsu
#2) If you are hit with a jutsu then your opponent will be injuring both you and your summon(killing 2 birdies with 1 stone)
#3) You become ugly as shit xd
The 3rd disadvantage was just for shiz and giggles xd
How you would use it in battle: I wouldn't use it but if I did then I would use it when the summonings abilities has an advantage on the terrain(eg. Toads on a water terrain)
How you would counter it: I would try to attack them while they are fusing(naruto and gamabunta = narubunta/gamato xd) or I would use jutsus that are powerful but quick to perform so if I do hit them, I will be injuring them both
How you perform it (with as much detail as you can): I bite my thumb to release some of my blood and perform the required hand seals before slamming my hand on the ground, summoning my Cheetah(made up) which stands next to me. I then let the Cheetah in on my plan and we begin focusing chakra throughout our entire bodies, we then release our chakras from our bodies to combine and make us become 1 being