If the two Roles have the same authority level, there's really no point in having two, since the authority level is the only point of Roles in Revolution. You should just call the Role SubAdmin. The only reason to have two roles is if you want to Admin A to have more or fewer permissions than Admin B when they are both in the same User Group.
That said, it does sound like it might be a bug in the way MODX handles Roles.
BTW, the authority levels are completely arbitrary, so if you really want two Roles just to have a different Role name as a reminder to you of who they are, you could just as easily use 1 and 2, 9 and 10, or any two numbers). As long as the users in the group have a Role with the minimum authority level set in the ACL entry, they'll get the specified Policy.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html