When the stand-by screen appears on both the V2 & V3, point their tops together, and press B (the middle button) on one of them.
If an message appears, what message was it?
If one of them shows the download/reset screen, that one's battery is low and needs to be replaced.
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To connect Tamagotchi P's and Tamagotchi iD L
On the Tamagotchi P's, the connection menu spans two pages. The options on the first page are (top to bottom):
Tamagotchi P's
Tamagotchi iD L
Download
Tama Deco Pierce
When "Tamagotchi iD L" is selected, another menu will appear. The options are (top to bottom):
Play
Propose
On the Tamagotchi iD L, the connection menu spans two pages. The options on the first page are (top to bottom):
Tamagotchi iD L
Tamagotchi iD
Tamagotchi Plus Color (TMGC+C)
Download
When "Tamagotchi iD L" is selected, another menu will appear. The options are (top to bottom):
Play
Exchange Item
Photo
Propose
On both the Tamagotchi P's and Tamagotchi iD L, select "Tamagotchi iD L", then select "Play".
When the "Stand By" screens appear (pink lettering), point their tops together and press the B (middle) button on one of them. The screens on both should change to show "Connecting" (red lettering), then after about 5 seconds, you should see them play a connection game, or one character jump over to visit the other.