You are certainly right that sometimes you read yeast descriptions and they all sound very good. If it was a bad yeast it would not be sold.
If you want to mix yeasts the best way to do it would be to save seperate jars of each washed yeast and add 1/2 of each to a starter for your next porter/stout. That would keep the amounts of each yeast the same at least to start. By the end of fermentation though, it would be impossible to tell how much of each is left, but if the beer tasted good, you could always then try washing that yeast cake and using it. After 1-2 batches, you would have a mystery population of 1028/1318 yeast that would still taste great and now be your "house" strain. To review:
1- Wash each batch individually, washing the yeast into either 1/2 or whole pint jars.
2- When you are going to brew your next porter or stout, take 1 jar of each and make your starter
3- Brew porter/stout with said starter
4- Wash yeast cake into pint jars
5- Enjoy brewing with your new house strain!