In my opinion, if you install the main bonding jumper and the connection to the GEC in the outside cabinet, then most inspectors would consider the outside cabinet only to be the service. In that case, double locknuts and plastic bushings would be fine for bonding the rigid nipple, because it is a feeder. Both the GEC and the main bonding jumper must connect to the same physical buss. The inside panel would have to have a separate neutral and ground bar because it is not the service.
However, if you install the main bonding jumper and the Grounding Electrode Conductor in the inside panel, then both cabinets constitute the service, which is also quite commonly done. No separate ground bar is required, though many people do install them. At least one bonding type lock nut or bonding bushing would then be required.
Another solution which many people use is a PVC nipple with panel adaptors and a coupling or male adaptors, locknut and plastic bushings. The grounded conductor (neutral) is still considered adequate for bonding the service cabinets if it exceeds the minimum size required for bonding.