Theme: D&D - Sorry boys, not Double D. Phrases that start with D's, more like Dungeons and Dragons except in this case, the first two letters of each D word are duplicated.
16A. First two reindeer named in Rudolph's song : DASHER AND DANCER
25A. Use a mouse to move a file between folders, say : DRAG AND DROP
43A. List of behavioral recommendations : DOs AND DON'Ts
55A. 2003 prequel subtitled "When Harry Met Lloyd" : DUMB AND DUMBERER
Argyle here. I don't know if having the first two letters dupe was intentional but it probably was. Still pretty straight forward for an Eric Agard puzzle. No need for a unifier or reveal. Outstanding long Down fill today plus two grid spanners.
Across:
6. Mess maker : SLOB. Certainly not Alfred.
10. Remote : FAR
13. Lightweight synthetic : ORLON
14. Nothing, in Nicaragua : NADA
15. Scheme in which three of four lines rhyme : AABA
19. Jai __ : ALAI
20. Fury : IRE
21. Baseball legend Mickey : MANTLE. Do you have a rookie card, C.C.? (From C.C.: Nope. Boomer had the whole set of 1952 Topps, but he sold all in the 1980s. He sold his Mantle card (in perfect mint condition) for $900 and has been regretting every day.)
22. It has a trunk but no wheels : TREE
24. Layered cookie : OREO
30. Queue between Q and U : RST. Unique way to clue an alphabet string.
33. Charged, infantry-style : RAN AT
34. The Beatles' "Abbey __" : ROAD. EMI Studios were on Abbey Road.
35. Administer, as justice, with "out" : METE
36. Eden exile : ADAM. 11D. 36-Across' second son : ABEL
37. Thorax organs : LUNGS. Thorax, the part between the neck and the abdomen.
38. Thor's father : ODIN
39. Book part : PAGE
40. Former Atlanta arena : OMNI
41. Lopsided : ATILT. An "A" word.
42. Make a typo : ERR
45. Cry of dismay : "ALAS!"
47. Ten-speed unit : GEAR
48. Prisoner : INMATE
50. "How can __ sure?" : I BE
51. Ring of light : HALO
58. Many Keats poems : ODEs
59. Stunt rider Knievel : EVEL
60. Sprinkles or drizzles : RAINS
61. Was in first : LED
62. "Don't touch that __!" : DIAL
63. Supplement : ADD TO
Down:
1. Sonata ending : CODA
2. Inland Asian sea : ARAL
3. "Casablanca" heroine : ILSA
4. Diamond gem : NO HIT GAME
5. Santa Barbara-to-Las Vegas dir. : E-N-E
6. Marching band percussion instruments : SNARE DRUMS
7. Freeway division : LANE
8. Unusual : ODD
9. Snits : BAD MOODS
10. Accounted for, as during calculations : FACTORED IN
12. Steak request : RARE
15. Diarist Frank : ANNE
17. Nothing, in Nice : RIEN
18. 50-and-over org. : AARP
23. Critter before or after pack : RAT
25. Fall in folds : DRAPE
26. Plane tracker : RADAR
27. Made "talent" from "latent," e.g. : ANAGRAMMED. Got thrown by the double M.
28. Prima __ : DONNA
29. 1980 De Niro film about a boxer : "RAGING BULL". Filmed in black and white.
31. Clown heightener : STILT. Neat trick, standing on one stilt.
32. Camp shelters : TENTS
35. British heavy metal band with the album "Ace of Spades" : MOTÖRHEAD
37. Not as tight as before : LOOSENED
41. Cavity filler's org. : ADA. (American Dental Association)
43. Census gathering : DATA
44. Regard : DEEM
46. Research sites : LABS
48. Revered entertainer : IDOL
49. Naked : NUDE
50. Inventor's spark : IDEA
52. Bone-dry : ARID
53. Gave for a while : LENT
54. Roughly : OR SO
56. 506, in old Rome : DVI
57. Bikini top : BRA. OK, Double D bra link
Argyle