Alright I'm going to try this again. Here is how I think a medallion is made.
Single Power Medallion
To start, you need a medallion or spike to grant aluminum Feruchemy to the person who has the power you wish to put in a medallion. This person is a willing participant. They store their Identity, and the power needed is spiked out.
You now have an unkeyed hemalurgic spike.
Next, you need a person who has, through whatever combination of Spikes, medallions, and natural ability, all of the following. Aluminum, Duralumin, and Nicrosil Feruchemy, and the allomantic ability corresponding to the type of spike that stores the power stolen.
This person Stores all of their Identity, and heavily taps Connection (whether this is a general thing, or a specific type of Connection I don't know. I don't think we know enough about it) imprinting their stores with Connection enabling the touch bond of the Medallions.
Identity free, and heavily Connected, this person burns the unkeyed Spike, and immediately stores what is produced in a nicrosilmind.
You now have a Hemalurgic charge, free of Identity and stored with connection to whoever holds it, tricking their spiritweb into believing that a spike has been properly placed to grant them the power (without the damage that normally occurs, avoiding the issue of hemalurgy's weakness).
Multiple Power Medallions and the Reason for the Three Power Limit
As you create these medallions, you'll eventually end up with a three power medallion that will give you all three types of Feruchemy needed to make them. No spikes needed in the maker.
At this point, you just need to burn and store multiple spikes. No problem.
Except for the limit of spikes that a human can have. Three spikes burned, three powers stored...and an attempt at a Fourth puts the maker above the threshold of control.
The Connection trick is why multiple medallions interfere with each other. By using Connection to convince the Spiritual Aspect that a spike has been placed, you create this limitation. Multiple medallions attempt to trick the Spiritweb in the same manner and it causes a mixed signal which interferes with both medallions.
It's like trying to listen to two radio stations broadcast over the same frequency. Instead of one clear signal, you get a garbled mess of unintelligible noise.
There are two main assumptions made here, that I know some people disagree with.
The nature of Nicrosil Feruchemy. Some people believe that Nicrosil is only capable of storing powers. I believe that Nicrosil can store any part of the Spiritweb, which is coincidentally what hemalurgy steals. If I'm incorrect, this method does not work.
Duralumin Feruchemy. Connection is not, in itself, Investiture. It is a property of, or force applied to, Investiture per this WoB.
So I believe that while tapping and storing Connection can have immediate effects, as seen with the translation medallions, I think it functions like Aluminum Feruchemy. It changes the level of Connection of the Investiture of the Spiritweb as a whole. As such that Connection, or lack thereof, would be preserved when the Investiture is stored, just as Identity is. Again, if this assumption is incorrect this method fails.
Edit: adding in two wobs to this. First, one I asked of Brandon recently
Doesn't tell us much, but it does show that the nicrosil in the medallions is not just standard Nicrosil Feruchemy.
Next this, which I found while looking for something else, and I think reiterates the point above, and supports the need for Connection manipulation.