Both of these soulmelds take up the waist body slot but I don't know if that matters since they're both soulmelds and the pheonix belt is only being shaped, not bound.
Look in Magic of Incarnum, page 38, for the feat:
Two of your shaped soulmelds can occupy (and be bound to) that chakra simultaneously.
Note: It requires Meldshaper Level 9th.
(Remember - What you want to do requires the feat mentioned above. Credit: Miniman.)
The meldshaper usually can't employ a phoenix belt and the manticore belt simultaneously
Magic of Incarnum in the chapter Soulmelds under Shaping Soulmelds under Meld Selection says that
[T]wo soulmelds can’t occupy the same chakra (with some exceptions). (50)
To realize a soulmeld's benefits, the soulmeld must be shaped (the soulmeld then occupying a chakra) or bound (the soulmeld then occupying both a chakra and a magic item slot).
Here are some of the exceptions mentioned in the quotation above.
- The general feat Double Chakra has as its prerequisite a meldshaper level of 9 and says that
When this feat is selected, choose a chakra to which you can bind soulmelds. Two of your shaped soulmelds can occupy (and be bound to) that chakra simultaneously. This counts as two chakra binds. (38)
A totemist who binds a soulmeld to his totem chakra has a soulmeld that occupies—instead of the totem chakra—another chakra:
When a totemist wishes to bind a soulmeld to her totem chakra, she must choose a different chakra for that soulmeld to occupy. Regardless of the chakra occupied by the soulmeld, however, the totemist can bind it to her totem chakra. This is an exception to the normal rule that a soulmeld can only be bound to the chakra it occupies. (51)
While this doesn't allow a totemist to have two soulmelds occupying the same chakra, it does allow a totemist to bind a soulmeld to one chakra and have that soulmeld merely occupy a different chakra. It's strongly implied that the totemist can pick any chakra to be occupied by his totem-bound soulmeld, but I can imagine a DM mandating a totem-bound soulmeld occupy one of the the soulmeld's default chakras (e.g. the brow chakra for a basilisk mask).
For example, under most readings a totemist could bind a manticore belt to his totem charka and pick the manticore belt to occupy his crown charka; simultaneously, the same totemist could bind a phoenix belt to his waist chakra.
The totemist's special totem chakra (double bind) says
At 11th level, you gain the ability to bind a single soulmeld to your totem chakra and to another chakra at the same time. You gain the special benefits of both chakra binds. (30)
While this, too, doesn't allow a totemist to have two soulmelds occupying the same chakra, it allows a totemist to bind a soulmeld to two different chakras if one chakra is his totem chakra.
Obliquely, this also means the same soulmeld can't usually be bound to more than one chakra (e.g. once a basilisk mask is bound, a meldshaper can't bind a basilisk mask to a different chakra until the meldshaper unbinds the mask from the first chakra.)
Just in case you were wondering, the section Soulmelds under Magic and Incarnum says that
two of the same soulmeld can’t be shaped on the same character simultaneously. (52)
So no doing that, either.
The prestige class incarnum blade at level 5 gains the special dual chakra binding (MoI 123), but such a character can only bind his special soulmeld blademeld to two different chakras, therefore receiving the benefits for having done both.
I am unaware of further exceptions.