library(sf)

## Linking to GEOS 3.7.1, GDAL 2.4.0, PROJ 5.2.0

library(nngeo)
library(mapview)

# Read CSV
dat1 = read.csv("dat1.csv", stringsAsFactors = FALSE)
dat2 = read.csv("dat2.csv", stringsAsFactors = FALSE)

# To layers
dat1 = st_as_sf(dat1, coords = c("x", "y"), crs = 32636)
dat2 = st_as_sf(dat2, coords = c("x", "y"), crs = 32636)
dat1

## Simple feature collection with 15 features and 2 fields
## geometry type:  POINT
## dimension:      XY
## bbox:           xmin: 603569.8 ymin: 3399802 xmax: 785825.2 ymax: 3590244
## epsg (SRID):    32636
## proj4string:    +proj=utm +zone=36 +datum=WGS84 +units=m +no_defs
## First 10 features:
##    id1 value1                 geometry
## 1    1      9 POINT (651974.1 3571355)
## 2    2     83 POINT (752119.6 3440607)
## 3    3     36 POINT (676253.9 3399802)
## 4    4     78   POINT (771062 3456974)
## 5    5     81   POINT (782552 3582291)
## 6    6     43 POINT (603569.8 3569298)
## 7    7     76 POINT (700079.6 3529951)
## 8    8     15 POINT (772942.4 3519491)
## 9    9     32 POINT (704745.5 3590244)
## 10  10      7 POINT (685781.5 3522531)

dat2

## Simple feature collection with 10 features and 2 fields
## geometry type:  POINT
## dimension:      XY
## bbox:           xmin: 603624.8 ymin: 3410358 xmax: 773467.6 ymax: 3554318
## epsg (SRID):    32636
## proj4string:    +proj=utm +zone=36 +datum=WGS84 +units=m +no_defs
##    id2 value2                 geometry
## 1    1    338 POINT (603624.8 3524413)
## 2    2    321 POINT (682898.6 3410358)
## 3    3    379 POINT (754243.5 3468184)
## 4    4    341 POINT (618838.4 3446267)
## 5    5    347 POINT (706648.1 3554318)
## 6    6    390 POINT (635764.8 3481093)
## 7    7    360 POINT (619964.9 3553403)
## 8    8    395 POINT (745120.1 3553868)
## 9    9    316 POINT (773467.6 3550258)
## 10  10    394 POINT (669351.1 3479356)

# Plot input layers
mapview(dat1) + mapview(dat2, col.regions = "red")

# Spatial join: nearest point from 'dat2' for each point in 'dat1'
dat = st_join(dat1, dat2, join = st_nn, progress = FALSE)
dat

## Simple feature collection with 15 features and 4 fields
## geometry type:  POINT
## dimension:      XY
## bbox:           xmin: 603569.8 ymin: 3399802 xmax: 785825.2 ymax: 3590244
## epsg (SRID):    32636
## proj4string:    +proj=utm +zone=36 +datum=WGS84 +units=m +no_defs
## First 10 features:
##    id1 value1 id2 value2                 geometry
## 1    1      9   7    360 POINT (651974.1 3571355)
## 2    2     83   3    379 POINT (752119.6 3440607)
## 3    3     36   2    321 POINT (676253.9 3399802)
## 4    4     78   3    379   POINT (771062 3456974)
## 5    5     81   9    316   POINT (782552 3582291)
## 6    6     43   7    360 POINT (603569.8 3569298)
## 7    7     76   5    347 POINT (700079.6 3529951)
## 8    8     15   9    316 POINT (772942.4 3519491)
## 9    9     32   5    347 POINT (704745.5 3590244)
## 10  10      7   5    347 POINT (685781.5 3522531)

# Plot lines between nearest points
x = st_connect(dat1, dat2, progress = FALSE)
mapview(dat1) + mapview(dat2, col.regions = "red") + mapview(x)