Not my best work, but I can't think of something unique for this pairing/show, so this'll have to do.
A young girl slowly opened the gate door to her friend's yard, only to be greeted by the two brothers.
The two brothers were up to their normal tasks, as usual. She wasn't sure what they were up to, usually they surprised her. So she did what she had done many times before and opened her mouth to pop the question.
"Whatcha doin'?" She flashed her girly grin at one boy in particular.
"Oh hey Isabella! Just building a bounce house." He warmly smiled back at her, oblivious to her small hints of attraction.
"Hey, um, Phineas? There's a dance tonight, and, well, I was wondering if-"
"Of course!" He put down the blue prints he had been working on. "What time is it?" He questioned her.
"Around 8 tonight." She stated, shocked.
"Ok, well I'll be prepared by then." He then picked up the papers again.
"Oh great! When do you think we can-"
"And I'll make sure Ferb's ready by then too!" He turned away, walking back to his scheme.
Her smile was no longer visible, as only a frown was present upon her face.