Technically, what you want to do is not possible because you get 1 set of MX records per domain and right now your MX records are pointed at the icloud email server, so all of the email that comes through that mx record goes to the icloud server and then the email addresses/inboxes are managed there.
Fucntionally, however I can think of a couple of solutions:
1) Slightly more challenging, but the cleanest solution would be to create a subdomain. so in this case you would have parnter1@
customdomain.com and patner2@
customdomain.com and point your MX for
customdomain.com to Apples servers and create your mailbox there. Then create a subdomain for instance shared.customdomain.com ... then shared.customdomain.com would have it's own set of MX records that you would want to set up with email forwarding .. so for instance you sould have
sharedmailbox@shared.customdomain.com forward to
partner1@customdomain.com and
partner2@customdomain.com
2) Not exactly the answer to your question but slightly easier would be to just register a second domain such as sharedcustomdomain.com and forward
shared@sharedcustomdomain.com to your email addresses. this would cost slightly more $ but would be less config then option 1.
Namecheap provides free email forwarding. I have not tried the config mentioend in option 1 on namecheap but im curious if it works.
personally, I use option 2...
my wife uses a gmail account
i have a custom domain hosted throug icloud mail
and then my wife and i have a domain for some rental properties that we own.
my set up is that i have
house1...n@rentaldomain.com forward to both of our email addresses and then i host the domain which is more related to my name on icloud and point my mx records there.
Please post back if you go with option1. im curious if it works !