A second red-eyed tree frog calls for a sensible step up in terrarium size, not a doubling of everything. Red-eyed tree frogs are small, nocturnal, and strongly arboreal, and two of similar size generally share a planted vertical enclosure peacefully, sleeping on leaves by day and hunting at night. The upgrade is about headroom, perching choice, and feeding access: give the pair more height and width than a single frog needs, and duplicate the specific spots each frog will want to claim after dark.
How two red-eyed tree frogs actually use the space
By day, each red-eyed tree frog sleeps flattened against the underside of a broad leaf, so the terrarium needs enough large-leaved plants that both frogs can find a sheltered leaf without sharing one. By night, both become active hunters moving through the branches, which means the vertical routes matter as much as floor area: overlapping branches, vines, and plant stems at several heights let the two frogs pass each other without either being cornered. A taller enclosure with dense planting serves a pair better than a wide, bare one, because the frogs live in the canopy layer, not on the ground.
Resources to duplicate for the second frog
Add a second feeding station rather than enlarging the first. A feeding dish or cup placed at a different height and position stops the bolder frog from sitting over the only insects and taking every item, and it lets you see roughly how much each frog is eating. Keep a shallow water dish both frogs can climb into, positioned where nightly misting keeps it fresh, and make sure misting coverage reaches the whole planted volume so neither frog's sleeping leaf dries out. Extra broad-leaved plants are a resource in themselves for this species, since each new plant adds sleeping sites, hunting perches, and the humidity-holding cover red-eyed tree frogs rely on.
Size matching and pairing rules
Match the two red-eyed tree frogs closely in body size before they ever share a terrarium. A noticeably larger frog will reach food faster, take the best sleeping leaves, and can stress a smaller companion into hiding and skipping meals. Introduce the second frog only after a quarantine period in a simple separate setup, where its appetite and droppings can be observed and any illness shows itself before it can reach the resident frog. Two males will call at night, which some keepers enjoy and light sleepers should consider before pairing, since a calling frog is a healthy frog and not something to discourage.
Feeding two nocturnal hunters fairly
Offer insects at dusk when both red-eyed tree frogs are waking, and split the feed between the two stations so each frog hunts its own patch. Watch the first several feeds end to end: the healthy pattern is both frogs striking at insects within the same session, while one frog repeatedly arriving late or retreating suggests it needs the stations placed farther apart or a few feeds offered alone. Dusting and gut-loading the insects matters twice as much with two mouths to feed, because a shy frog eating fewer, poorly prepared insects falls behind nutritionally faster than its weight change first suggests.
Signs the shared terrarium needs adjusting
Adjust the setup if one red-eyed tree frog stops appearing at night, loses weight around the hips, or spends the day on the glass or floor instead of a leaf, since ground-sitting by day can signal stress or illness in this arboreal species. A veterinarian with amphibian experience should see any frog that goes several nights without eating or shows skin discolouration. When both frogs sleep on leaves, hunt eagerly, and hold their weight, the pairing and the space are right. Logging the pair's misting times, station refills, and nightly feeding notes through KinsLe Family Sharing keeps every helper working to the same routine for the terrarium.