Having It Both Ways solution
by Kegler

Each shared square contains two letters, to be read in one order for the Across entry and in the    
other order for the Down entry. In the grid below, the letters to the left of the backslashes are read
first for the Acrosses, and the letters to the right of the backslashes are read first for the Downs.    

Having It Both Ways solution grid

Across
1A(L + DER)MEN (RED rev)
4EDI(T)S (SIDE rev)
6ET(HERE)AL
9NEW WORLD ("GNU WHIRLED" hom)
10IRONICAL (OIL IN CAR anag)
11ARDOR (hidden)
12DE(IG)N (G.I. rev)
13REIgNED
15RECAP (PACER rev)
17EPISTLES (PILES SET anag)
19P + ALMET + TO (METAL anag)
20E(YES)IGHT
21NADIR (DRAIN anag)
23L + ABE + L
24RESOLV + E + D (LOVERS anag)
25SU + SPIRES
26RIG + A + TON + I
28DEMO + N
30B + EDEWS (SWEDE rev)
32I + MAGE (GAME anag)
34INURE (rev hidden)
35CO + MEDIAN
37TOLLGATE (ALL TO GET anag)
39SNIF + FLE + R (rev)
40BERYL (hidden)
41T + SARI(ST)S
Down
1LAV(END)ER
2RESOW (hidden)
3NEED + LED
4DERRI(N + G)ERe
5S(T.A.)IN
6TESLA (TALES anag)
7RE(TRAC)TS (ST(CART)ERn rev)
8L + ACROSS + E
14ENSEMBLE (MEL'S BEEN anag)
16PALISADE (hidden)
17exPEN(THOU)SE
18COTTER PIN ("POTTER KIN" spoonerism)
19APPA + L (PAPA anag)
20Y + mELLOw + WE'D
21AN + TI(POD)E
22RINSE (SIREN anag)
26I + RAN + IAN'S
27TAPER OFF (rev hidden)
29NO(MIN) + ALLY
31S + WEE + TEST
33EGOTIST (SITE GOT anag)
35O + CHRE (CHER anag)
36DE(NEd)B
38GLAIR ("GLARE" hom)