Cross-Database Query Without Casting to List

I am trying to do a cross-database query without having to enumerate the variable t.

public List<Models.MCMessageCenter> GetMsgCtrsWithReloInfoByUserId(Guid userId)
{
    var t = GetMessageCentersByUserId(userId).ToList();

    List<Models.MCMessageCenter> result;

    using (var ctx = new TEDREntities())
    {
        result = (from n in t
                 join m in ctx.UserAccounts.AsNoTracking() on n.UserId equals m.UserAccountId
                 select new Models.MCMessageCenter
                 {
                     Id = n.Id,
                     MessageTitle = n.MessageTitle,
                     CreatedDate = n.CreatedDate,
                     UserId = n.UserId,
                     Name = m.FirstName + " " + m.LastName,
                     RelatedEntityId = n.RelatedEntityId
                 }).ToList();
    }

t could be very large so I don't want to enumerate it by turning it into a list. Is there a better, more efficient way to perform a cross database query?

Whenever I attempt to keep t as IQueryable, I get an error like "can't query two datasources at once." I can get the exact message if anyone needs.