Bankei wrote:I wonder why monks of one nikaya won't hold any sanghakamma, such as Patimokkha or ordination ceremonies, with members of another nikaya. For example in Thailand members of the Mahanikaya and Dhammayutika nikaya will jointly chant at funerals etc, but they will not hold sanghakamma together.
Funeral ceremonies aren't really a relevant example; they are not sanghakammas and have no Vinaya significance at all.
Regarding the situation between the two nikayas in Thailand, I don't think it is readily describable in Vinaya terms. As far as I know neither nikaya has anything like an official position statement as to how it regards the monks of the other nikaya (i.e., that they are schismatics, or not really monks, or whatever). Prince Mongkut's personal doubt about the intactness of the Mahanikaya ordination lineage doesn't have any status as a Vinaya ajudication, even if some in the Dhammayuttika Nikaya take it seriously.
In practice the monks in a Mahanikaya monastery will normally allow a visiting Dhammayutt monk full participation in sangha activities, while Dhammayutt wats are highly inconsistent in how they treat visiting Mahanikaya monks. For example, the extreme chauvinists among the Dhammayutts (e.g., Ajahn Maha Boowa and his associates) will treat a visiting Mahanikaya monk like a samanera. On the other hand, in the north of Thailand (where the Dhammayuttika Nikaya is sparsely represented) many Dhammayutt forest ajahns don't care a damn about nikaya affiliation.
When I was a Dhammayutt monk myself I remember sometimes hearing whisperings about "the regulations of the Dhammayuttika Nikaya". For example, Mahanikai monks can stay at Dhammayutt wats, but must leave during the rains retreat; Dhammayutt monks can confess to Mahanikai monks, but can't hold the Patimokkha recital with them, etc. But these regulations seem to be very elusive. Nobody could ever show me a copy of them, or give me a citation; nor have I ever met a monk who admits to having seen a copy of them.