Longoria, who recently split from boyfriend Tony Parker, and Lopez are old friends.
Arriving four minutes to show time, the Desperate Housewives star was shuttled to her seat in the front row next to Lopez's mother. Since producers had banned clothes with messages of support for contestants, Longoria concealed her homemade black "Vote For Mario" tee (which she'd Bedazzled herself!) under a tan blazer.
After the show, Lopez told PEOPLE, "We were told that our families couldn't wear the t-shirts. I called (Eva) and told her and she said, 'Well, it's too late. I already made it and I'm wearing it.' And I said, 'Okay, but you're breaking the rules this week, not me!' " (Last week, Lopez and partner Karina Smirnoff were chastised for breaking their hold and doing a lift during their routine.)
Lopez said he'd known Longoria "for years, yeah. She's like a sister." Having her at the show, he said, "was great. She was supposed to come other weeks but she's busy, plus we tape kind of early because we're live to the East Coast, so it's hard schedule-wise. She's got her show going, you know."
Although Lopez and Smirnoff deny they're a couple ("A couple of nuts!" Lopez joked; added Smirnoff, "He stepped on my hand, sat on my head. How do you put that together? I am crazy and weird, but not that weird!"), the partners have obvious chemistry off the dance floor.
On their performance Tuesday, Smirnoff told PEOPLE, "The music is from such a powerful movie, Desperado. We really aimed to be Antonio Banderas and Salma Hayek tonight. I hope it came across."
"(Salma Hayek) wishes she looked as good as Karina did tonight," said Lopez.
Replied a smiling Smirnoff: "Vice-versa, really."