Well, there are several problems with that...
- You try to use "tamte" the way as "to" is used with or instead of the verb "być": https://forum.duolingo.com/comment/26090996 §2. But only the singular form of "to" can be used that way - for both nouns in plural and singular form. You may say, that "to" is the only dummy pronoun in Polish. And when "to" is used with plural, it is only a replacement for the grammatically proper forms "te/ci" and "tamte/tamci".
- However, the grammatically correct forms "te/ci" and "tamte/tamci" force the use of instrumental case (while you can use nominative with "to"). Also many such forms, especially when used with nouns alone in the plural, are actually archaic. They can be sometimes used with noun phrases (a noun with an adjective or possessive pronoun), but that is a blurred area, you may sometimes sound strange when you use them that way.
- Besides, "tamten/tamta/tamto/tamte/tamci" is not exactly "that" in English. Its meaning is rather closer to "yonder", except that it is archaic in English, but not in Polish. A detailed explanation is here: https://forum.duolingo.com/comment/26090156 .
- Due to the above, "tamten/tamta/tamto/tamte/tamci" is not quite used to point out things, only to differentiate between them. So, in general, you can use "tamten/tamta/tamto/tamte/tamci" only if you have used already "ten/ta/to/te/ci" in the same sentence or in a previous one. But you do not quite use "tamten/tamta/tamto/tamte/tamci" out of thin air. In such cases you rather use "ten/ta/to/te/ci". And it is mostly used with adjectives or with other words that describe things - not quite with nouns. Eg:.
- Te koszule są moje, a tamte (są) twoje. = These shirts are mine and those are yours.
- Podoba mi się ten dom, ale tamten jest tańszy. = I like this house but that one is cheaper.
- Ten kwiat jest żółty, a tamten czerwony. = This flower is yellow and that one is red.
- To jest zabawka, a tamto - żywe zwierzę. = This is a toy and that is a living animal. - ATTENTION, risky ground. You cannot repeat the verb but you need to replace it with a dash, and even then sometimes your sentence will sound off or childish.
- You generally do not use "tamten/tamta/tamto/tamte/tamci" as an indicative pronoun with a noun and a verb. If you think you need to use "tamten/tamta/tamto/tamte/tamci" as an indicative pronoun for something distant, you'd rather rebuild the sentence so that you use the adverb "tam" (= "there; out there") together with the proper verb.
- To są moje koszule, a tam są twoje = They are my shirts and yours are out there.
- Podoba mi się ten dom, ale tam jest tańszy = I like this house but the one out there is cheaper.
- Ten kwiat jest żółty a tam jest czerwony = This flower is yellow and there is a red one out there.
- To jest zabawka, a tam jest żywe zwierzę. = This is a toy and there is a living animal out there.