I recently read an article about an OIS between a DEA agent and a drug dealer. The agent was armed with a 1911 loaded with 4 rounds of .45 ACP JHP's and 3 rounds of FMJ ball. The dealer fled and made it into a vehicle where he was subsequently killed by a FMJ bullet which impacted his melon. According to the agent, the JHP's would not reliably penetrate the vehicle. This is suspicious to me, but I wasn't there and wasn't privvy to what JHP he was using. Its not impossible, but slightly improbably. The officer went on to say he choose this obscure loading for this very reason and advocated the same for his colleagues.
That being said, I think this is a bad idea, especially mixing 55grn JHP with 64grn JSPs. Both rounds will have different points of impact and feeding properties. Weapons are complex mechanical devices and certainly don't need one more thing which could cause a failure. Its much better to pick a single defensive round suited for your weapon/environment and supplement this with an auxillery round. Select your round based on your weapon, understanding of terminal ballistics and environment/threat. 75grn TAP is a great choice for an all around defensive bullet.
For an auxillery load, a bonded JSP is an excellent choice. Having two magazines loaded with this ammunition is an excellent idea as thier barrier performance is superior. No cartridge will be effective in every situation, so you must make compromises and hedging your bets with an auxillery load is a great idea, albiet in a different magazine. Tracers are the exception here as they are both FMJ's and of similar weight, provided you are using corresponding ball/tracer loads.
I too "Dutch" my shotgun. I have a Remington 870 Police with a side saddle, Speed Feed IV, 5 round but cuff and Blackhawk bandoleer sling. When out in the field, I load it with Remington standard pressure 000 Buck and Remington low recoil 1 oz slugs, both 2 3/4. This gives you an incredible amount of firepower and versatility. I alternate buck/slug in the magazine, saddle, sling and but cuff so as to have an equal amount of both rounds. Indoors, this is extremely dangrous as it has a gross amount of penetration. If you live in the country, this is probably an excellent choice for HD, for those who live in urban areas, its most reckless. When indoors, I take it off "11" and load the side saddle and magazine with 2 3/4 Federal BBB with 5 rounds of Hydrashok slugs in the butt cuff, just in case. Mixing two different sizes of shot in the weapon is senseless, if anything its a handycap, for example alternating 1 and 000 buck or 6shot and 00buck. Shotguns and tracers not with standing, its best to pick a singular defensive round to maximize your capability in a large number of scenarios rather than mix rounds in a magazine and create possible failures. It seems like a reasonable idea, however its not.....