This uses the Mark Harris MPE legacy block (https://www.native-instruments.com/en/reaktor-community/reaktor-user-library/entry/show/8899/).

Roli Seaboard Setup: MPE preset with No. MIDI Channels = 2
(this transmits on Midi channels 2 and 3)

Two versions are include in the zip file.
a). Petras – MPE Duophonic Synth. As the MPE block has no velocity, this patch attempt to create it using the Note In blocks, one set to channel 2 and the other channel 3. NOTE!! These channels are not remember when saving the rack or ensemble depite the Connection properties being set to the correct channels. This requires assigning the channel numbers each time for each Note In block, or else you will hear nothing.
b). Petras – MPE Duophonic Synth No Velocity. This is the hassle free version without velocity.

The download zip files include both the rack (.nksr) and ensemble (.ens).

Reaktor version used: 6.5.0

My Reaktor paths:
Documents Native Instruments/Reaktor 6/Blocks/MPE/MPE_MODULE_WITH_PB.mdl
Documents Native Instruments/Reaktor 6/Blocks/Petras/Racks