@AngeloF I presume it is actually an MX105 you are referring to? As mentioned above you cannot have more than one active MX in a network and the passive has to be the same model as the active. However you can have more than one MX in a site, just create a second network.
For all of our sites we have at least three MXs and divide them into multiple networks to suit our needs.
What are you trying to achieve?