Running something concurrent means that any jail or prison time would be served at the same time. If your fiance was convicted of both charges, or plead guilty to both charges then you would want an agreement or argue to the court that any time he might serve would run concurrent with the other sentence. This would reduce the amount of time in jail or prison.
This assumes alot, I have no idea about the merits of this case so I am not suggesting that he plead guilty or proceed to trial. He should consult an experienced attorney in your area for more detailed assistance. There may be a statute that prevents these types of charges from running concurrent. Ask an attorney in your area.
To put it simply, an example of a concurrent sentence would be:
if he received 25 years on each of the precursor charges, then consecutive sentences would mean a total of 50 years, 25+25, he would serve the first sentence and then begin serving the second sentence. This is not very common but it is inceasing in popularity it seems. Concurrent sentencing would mean each of the 25 year sentences woudl run together meaning that he would serve one 25 year sentence and each year would count aas a year on each of the precursor charges.
Speak to your fiancee's lawyer about this so that it can be part of any plea discussions with the District Attorney's office. I am sure that the lawyer will try to do this anyway, but be sure to discuss with her/him.
Having multiple charges pending from separate arrest can sometimes be overwhelming to individuals who have not been though this process. From the information you provided, your fiancée is facing substantial jail time.
My first recommendation is to make sure he has an attorney working for his best interest. If he is considering pleading guilty to more than one charge (which I am not advising since I do not know anything about the cases) he would need to talk to the district attorney handling his case and discuss options for a plea recommendation. The district attorney recommendation can include running any jail time concurrent (together) with each other. This would allow for him to be release much sooner than if he had to serve them consecutively (back to back).
Nevertheless, should your fiancée consider pleading to some of the charges, he needs to be very informed of the consequences. Some of the offenses carry mandatory 85% of the time to be served, and if he is convicted of enhanced drug offenses (meaning he has already been convicted of a prior drug offense), there is potential for doubling the penalty.
Again, I would advise him, as well as you, to consult his attorney to discuss this matter in detail prior to deciding what avenue he is going to choose.