Nice crossover with both shows together. Not a big fan of Johnny Test, but I enjoyed this crossover. It was funny when Buford told Baljeet that Susan and Mary look nerdier than him. LOL

This needs a sequel.

No está mal.
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Not bad.

Doofenshmirtz atrapó a Perry y Dukey.
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Doofenshmirtz trapped Perry and Dukey.

Not a bad crossover.

Dude... that...was...AWESOME!
this is one of the BEST crossovers I've ever read! *clapping until my hand gets purple*

Nice!

Nice!

Yum, a giant cookie! :D
Candice: "Mom! Mom! Phineas and Ferb made a giant cookie! And met up with a kid from another dimension!"
Candice's mom: "That's nice sweetie."
I'm a little surprised Susan and Mary didn't expect Johnny to use the time machine. I mean, they went to get lunch and left Johnny alone in their room. That's practically screaming for trouble. You don't have to change it, I think your story works either way, but I think Susan and Mary should have at least picked up on that. If you change either this chapter or chapter one maybe you could have had Susan and Mary kick Johnny out of their room before they went to get a snack. Or another idea that works instead is that you could have made it so Susan and Mary set Johnny up to use the invention. Maybe they wanted someone to test it out (Or an excuse to get rid of their brother for a bit without getting in trouble lol) so they tricked Johnny into using the invention by himself. The only problem with that idea is that it may change the plot with future chapters. I'll have to read on to see what your plan is.

One thing that I like about this chapter is that it's really easy to picture the scenes. I also can't help reading everybody's lines in their voice. XD
I like the idea Dukey going with Perry on a mission and how he agreed to do it because Perry offered him beef steak. One thing that confused me though was the part where Perry was talking to Major Monogram, Dukey didn't talk at all. I thought he may have been listening to them quietly (which is unusual for him), but then when Perry got onto his flying car, it was obvious that Dukey somehow missed the whole conversation. So I'm confused what happened to him. Maybe he wasn't paying attention because he was roaming the lab (Or whatever you'd call that area lol.)
Also I like the humor you add. Some of it is pretty subtle, but it fits well with the characters. Any humor you can add in a story like this is a must because it always makes the story better. :)
This is just random, but when Phineas and Johnny first met, I half expected Johnny to mention the winter or something and then you could have Phineas ask, "What's that?" XD Even though it's not entirely true to the series, it's fun to have characters poke fun at themselves or their world. As long as you're not bashing and it's all for the love of the series, fans will respond well to it. :)

Here are my thoughts on chapter one! :D
LOL! And of course if you tell Johnny to stay away from the invention he's going to test it out. XD (Wonder if that's why the creator of the show named him Johnny Test.) This brings back memories for me because I used to be a fan of Johnny Test as a kid. I've sort of grown out of it, but your story is helping me remember how much I liked it. :P
Anyways, I like this so far. Your writing isn't overly descriptive, but it's not so under explained that it's confusing. I've never had much description in my writing either, but honestly I like your way of story telling best.
I can't really comment on the plot yet because so far it has been pretty predictable, (Which you'd expect), there are only so many ways for Johnny and Dukey to wind up in a different universe and I'd probably be outraged if they didn't take the opportunity to test the invention. XD
What I'm really curious about is how the plot will change once Johnny meets Phineas and Ferb. :D

Awesome Story

Great story. Loved how you combined both shows but managed to keep each show's own flavor separate. It kept my interest and it was funny

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Cool story!

I wish they'd make an actual Johnny Test/Phineas & Ferb crossover on TV.

Nice story. You should write a sequel with Phineas and Ferb visiting Porkbelly (but hopefully for Perry, Johnny will keep the information about him being a secret agent to himself).