I don't have any Linux machines in my database, so I can't test to make sure it's 100%, but in theory this should work. If it doesn't, strip off the Windows part and adjust the Linux part to produce the expected results, then put the two together with the UNION again.
-- the original Windows-centric query
Select Top 1000000
  tblAssets.AssetID,
  tblAssets.AssetName,
  tblAssets.Domain,
  tblAssets.IPAddress,
  tsysIPLocations.IPLocation,
  tblAssetCustom.Manufacturer,
  tblAssetCustom.Model,
  tsysOS.OSname As OS,
  Coalesce(tsysOS.Image, tsysAssetTypes.AssetTypeIcon10) As icon,
  Convert(nVARCHAR(10),tblBIOS.ReleaseDate,101) As [BIOS Date],
  Cast(Round(DateDiff(day, tblBIOS.ReleaseDate, GetDate()) / 365.00, 2) As Numeric(8,2)) As [Years Old],
  tblAssets.Lastseen,
  tblAssets.Lasttried
From
  tblAssets
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
  Left Join tsysIPLocations On tsysIPLocations.LocationID = tblAssets.LocationID
  Inner Join tblState On tblState.State = tblAssetCustom.State
  Left Join tsysOS On tsysOS.OScode = tblAssets.OScode
  Inner Join tblBIOS On tblAssets.AssetID = tblBIOS.AssetID
Where
  tblState.Statename = 'Active'
UNION
-- a copy, modified to pull from tblLinuxBIOS instead of tblBIOS
Select Top 1000000
  tblAssets.AssetID,
  tblAssets.AssetName,
  tblAssets.Domain,
  tblAssets.IPAddress,
  tsysIPLocations.IPLocation,
  tblAssetCustom.Manufacturer,
  tblAssetCustom.Model,
  tsysOS.OSname As OS,
  Coalesce(tsysOS.Image, tsysAssetTypes.AssetTypeIcon10) As icon,
  Convert(nVARCHAR(10),tblLinuxBIOS.ReleaseDate,101) As [BIOS Date],
  Cast(Round(DateDiff(day, tblLinuxBIOS.ReleaseDate, GetDate()) / 365.00, 2) As Numeric(8,2)) As [Years Old],
  tblAssets.Lastseen,
  tblAssets.Lasttried
From
  tblAssets
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
  Left Join tsysIPLocations On tsysIPLocations.LocationID = tblAssets.LocationID
  Inner Join tblState On tblState.State = tblAssetCustom.State
  Left Join tsysOS On tsysOS.OScode = tblAssets.OScode
  Inner Join tblLinuxBIOS On tblAssets.AssetID = tblLinuxBIOS.AssetID
Where
  tblState.Statename = 'Active'
Order By
  [Years Old] Desc