Learn Finnish: Luin pienet kirjaimet molemmilla linsseillä. - I read the small letters with both lenses.

The base form is molemmat. To add most plural case endings, we use its plural stem. Because the adessive ending -lla closes the syllable, consonant gradation changes the strong stem -mpi- into the weak stem -mmi-. Adding -lla to this weak stem gives us molemmilla.

Why is the object pienet kirjaimet ending in -t rather than in the partitive case?

Because the action is complete. The speaker successfully read the letters. For a fully completed action with a plural object, Finnish uses the nominative plural form (which acts as the plural accusative), ending in -t. If the reading was ongoing or incomplete, it would be in the partitive: pieniä kirjaimia.

How is the plural adessive formed for the word linssi (lens)?

For relatively new words or loanwords ending in -i like linssi, adding the plural marker -i- causes the original stem vowel -i to change to -e-. This gives the plural stem linssei-. Then you just add the adessive ending -llä (meaning 'with') to get linsseillä.

Does molemmilla have to match the case of linsseillä?