The requirements would depend on the utility along with what your willing to spend on protective coordination. A double feeder scheme can be either have an open or closed tie breaker as well as make before break or always break before make transition. Operating in a closed loop is always far more complicated then an open loop.
In an open loop with break before make things are easy and the sources do not have to have an identical phase angle, frequency voltage ect. A loss of one supply circuit will cause your normally closed source sectionalizing breaker to open on a loss of voltage response while the tie point breaker then closes after the source breaker opens restoring the buss. The tie breaker is usually coordinated to trip first should a buss fault occur or the buss its closing into is faulted. Interlocks are put in place to prevent the tie breaker from closing into a live buss as well as while the source breaker is still closed. Once the source supply feeder gets restored the tie quickly opens and the source sectionalizing breaker closes. There is a brief power outage during this transition. This type of scheme is the simplest and perhaps the most reliable plus its definitely something pocos prefer for customer owned feeders.
The other is a normally closed scheme and it can get complicated. Both feeders need to be in near perfect synchronization, no large amount of current can flow between them when the tie breaker is closed. These schemes are the easiest to coordinate generally when both feeders originate from the same poco substation buss; ie, in essence the feeder cables are running paralleled. Fault current is higher both in the feeder cables and all branches originating from the customer owner gear, this is especially true if both feeders are fed from separate poco transformers. Relaying is done thorough differential comparison logic. Cts measure and compare directional flows of current and initiate a trip command to the two breakers between the fault when the abnormality takes place. Getting this logic to work right takes good analysis. Often the poco needs to know about this type of set up as well since when they are coordinating their breakers they need to be aware of how to take that closed scheme into account. Usually their engineers will work in conjunction with you so nothing gets missed. If other loads are attached to the feeders before they reach the customer the poco may discourage the idea since it can increase the fault current on their own equipment. POCO may also discourage it if they have to add anti reverse flow logic to their reclosers or breakers, since normally pocos operate MV in radial not anticipating reversed flows. (the exception is large city networks but back flow is prevented via a network protector on the LV side of the step down transformer)
However, when done right the only interruption experienced is the voltage sag from a cable fault or protective devices in the substation clearing from faulted equipment. Once the feeder is restored there is no transfer interruption, which is beneficial to data centers, hospitals and the like. Closed transition set ups are usually done on large facilities and its not uncommon for the poco to own the primary switch gear either though I have seen it go both ways.