Enroute Controller described a conflict event when an aircraft was descended to an occupied altitude during hold resulting in two aircraft momentarily holding at the same altitude; noting prior strip marking discipline has faded with URET implementation.

Narrative

[We had] bad weather night with extensive deviations; holding and diversions. [I was] holding aircraft at the VOR for one airport as well as over a second VOR for another airport. I was working position alone without D-Side. Holding patterns overlap; so I had to stagger altitudes. Working enroute and crossing traffic over; under and through altitude gaps in these two patterns. Traffic volume moderate; but given the holding configuration; complexity was high. When aircraft holding at FL220 diverted to alternate airport; I descended the wrong aircraft to the vacated altitude. I caught my mistake and promptly corrected it; however for a few moments two aircraft were holding at the same fix at the same altitude. Luckily; they were at opposite ends of the pattern and separation was not lost. Throughout my initial training and operation; holding aircraft was taken very seriously. Strip marking and strip board management was strongly emphasized. You could look at the strip bay and know immediately which aircraft had received holding instructions; what altitude he was at; what time he entered the hold and; if necessary; what the detailed holding instructions were and the alternate airport. After a crash event; the FAA took holding times very seriously and all delays were meticulously tracked. With the advent of URET; none of this is available. If you want to take the extensive time away from the scope to enter all of the holding information into URET you can; but it's a waste. Like many controllers; I now just put the EFC into the 4th line on aircraft holding. If I had strips; this mistake would not have occurred. Also if our airspace was configured to allow simultaneous holding without overlap this may not have occurred. Additionally; if I wasn't so busy working other traffic through; above; below and around these patterns; it may not have occurred.