Joel
Berestechko, Demidovka are in Dubna county Volyn province
A Revision list gets translated by Google as a "Fairy Tale"
Alex K has posted 1850 and 1858 revision lists for these towns - I have posted the links and you are in luck in that he has also posted the town detail within each list.  The numbers next to the town names represent the pdf page numbers where each town's data begins - so while these are in Cyrillic and as yet not translated - this will make it much easier to search for your family names.  

You can use the Steve Morse websites to show your family names in Cyrillic cursive and then compare.

English to Russian: Transliterating Names from English to Russian in One Step
https://stevemorse.org/russian/eng2rus.html
Russian Print to Cursive: Converting from Russian Print to Cursive in One Step
https://stevemorse.org/russian/cyrprintcurs.html?font=print

Dubna County [ ed. | ed. code ]

• The fairy tale of the Jews of Dubna County, Volyn Province. 1850  // DAJO. F. 118. Op. 14. Ref. 3.

1 Dubno; 362 Olika; 542 Varkovichi; 589 Mizoch; 622 Kozin; 653 Willow; 673 Krupets; 682 Mykhailivka; 704 Berestechko; 822 Merchant; 854 Ostrozhets; 882 Ant; 916 Mill; 929 Lyubochivka; 936 Demydivka ^[1]

• The fairy tale of the Jews of Dubna County, Volyn Province. 1858  // DAJO. F. 118. Op. 14. Ref. 8.

914 Lobachivka; 944 Берестечко; 1106 Demydivka; 1170-Mykhailivka (Buremel); 1242 Ant; 1358 Merchant; 1458 Willow; 1510 Kozin; 1610 Krupets; 1639 Olika; 2074 Mizoch; 2154 Varkovichi; 2274 Mills; 2236 Ostrozhets

Also additional revision list  118-14-79. Berestechko starting p250. Radzilov starting p 39, 121, 160, 181, 244, 3361862-1865 link:

Additional auditory tales of the Jews of Volyn province. 1862–1865  // RAIN. F. 118. Op. 14. Ref. 79.

For Radzivilov that is in the Kremenets district in Volyn 
Alex has posted revision lists from 1858 but unfortunately without details of towns.  Here are the links:

Kremenets district [ ed. | ed. code ]

• The fairy tale of the Jews of Kremenets County. 1858 // DAJO. F. 118. Op. 14. Ref. 144.
• The fairy tale of the Jews of Kremenets County. 1858 // DAJO. F. 118. Op. 14. Ref. 142.

you will also find records from Demidkova and Radzivlov in JRI-Poland database

hope this helps