I don't have an installation of SQL CE to experiment with, but give this a try. From the quick Google results, I gather that CE has problems with subqueries that return distinct values -- the Count(*) in the code I initially offered -- but it should be OK with subqueries that return sets of results. If that's the case, try joining the base "show me the NICs" query against itself in a subquery but specifically make the link against not-the-current-NIC.
It's not elegant, but it's a starting point if it works.
Select Top 1000000
tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblNetwork.IPAddress,
tblNetwork.Description
From
tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblNetwork On tblAssets.AssetID = tblNetwork.AssetID
And tblNetwork.IPAddress <> ''
And tblNetwork.IPAddress <> '0.0.0.0'
And tblNetwork.Description Not Like 'VMWare%'
And tblNetwork.Description Not Like '%virtua%'
And tblNetwork.Description Not Like '%Loopback%'
And tblNetwork.Description Not Like '%USB Remote NDIS Network Device%'
Inner Join ( SELECT
tblAssets.AssetID,
tblNetwork.IPAddress,
tblNetwork.Description
FROM
tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblNetwork On tblAssets.AssetID = tblNetwork.AssetID
And tblNetwork.IPAddress <> ''
And tblNetwork.IPAddress <> '0.0.0.0'
And tblNetwork.Description Not Like 'VMWare%'
And tblNetwork.Description Not Like '%virtua%'
And tblNetwork.Description Not Like '%Loopback%'
And tblNetwork.Description Not Like '%USB Remote NDIS Network Device%'
WHERE
tblAssetCustom.State = 1
) AS SecondNIC ON SecondNIC.AssetID = tblAssets.AssetID
AND SecondNIC.Description <> tblNetwork.Description
AND SecondNIC.IPAddress <> tblNetwork.IPAddress
Where tblAssetCustom.State = 1
Order By
tblAssets.AssetName,
tblNetwork.Description