PLTS OF 2 ACFT PLANNED A FORMATION FLT TO TAKE THE WINGMAN'S AG ACFT FOR SVC. IN A WX BRIEFING, VFR FLT WAS NOT RCMDD & THE PLTS WERE ADZD OF MODERATE RIME & MIXED ICING CONDS IN CLDS BLO 7000'. THE WINGMAN EXPRESSED A SENSE OF URGENCY TO DELIVER HIS ACFT & THE FORMATION FLT DEPD W/O A FLT PLAN. EN ROUTE, THE FLT LEADER (IN PIPER PA-32, N3212W) OBTAINED WX INFO, THEN ELECTED TO DIVERT TO MATTOON ARPT. HE SAID HE ELECTED TO DSCND THRU A 'SCATTERED' CLD LAYER & APPLIED CARB HEAT; HOWEVER, DURING DSCNT, THE ENG LOST PWR. SUBSEQUENTLY, THE ACFT COLLIDED WITH TREES & CRASHED. THE WIGMAN RPRTD THAT HE WAS ABT 2000' TO THE RGT AS THEY PENETRATED THE CLD LAYER & HE LOST SIGHT OF THE LEAD ACFT. ALSO, HE RPRTD HE ENCTRD SEVERE ICING CONDS & WAS 'VERY CLOSE TO THE GROUND' WHEN HE DSCNDD BLO THE CLDS. THE WINGMAN'S ACFT HAD NO RDO; AFTER DSCNDG BLO CLDS, HE COULD NOT FIND THE LEAD ACFT. HE WAS UNSURE OF HIS LOCATION, BUT LNDD SAFELY AFTER FLYING BY A WATER TWR AT MARSHALL. ABT 15 MI EAST AT TERRA HAUTE, IN, THE WX WAS IN PART: 1100' OVC, 5 MI VIS WITH HAZE & FOG.