try to replace https://myplatform/BOE/... in the child frame with https://myip/BOE/... and see if it works as well.
If so, then my assumption is that the session management of the BOE is not capable of hosting two applications in the same browser window using the same session ID. We have had the feedback, that BOE session management is on window basis, not on frame basis. So the iframe architecture might work with some applications but not with others.
In case my assumption is correct, then you could try to provide an SSO mechanism to both server URLs somehow (e.g. using SAP Portal or AD SSO) that essentially then would create two logon sessions that do not interact.
Best Regards
Jörg